Many valves are still available. A good range is being manufactured again, and are available from dealers. Often though they are aimed more at the audio market. Many of these originate from the former Soviet Union, and the developing countries.
There are still a lot of New-Old-Stock (NOS) valves appearing from old workshops and local TV and radio shops that are closing down. These tend to be available through specialist valve dealers like Valve and Tube Supplies on the Isle of Wight. Most of the "new" valves you order from such dealers will be NOS. Personally I would much rather use a NOS Mullard or Brimar or whoever valve, than a modern copy. New and NOS valves are normally guaranteed for three months, but this is void if the valve is damaged by a set fault.
If a new replacement valve is not available or is too expensive for your budget, one or two dealers sell used-tested valves. Valve and Tube Supplies have a vast range, with many costing less than five pounds. Due to their nature, used-tested valves may have only a minimal guarantee (typically one month), but I have had no problems getting the occasional faulty item exchanged.
Do not assume that all valves are available though. Some earlier valves can be more difficult to come by - which will be a problem if your interest is sets from the 20s and early 30s. In particular the original bright-emitter and early dull-emitter types in working condition are extremely rare and command high prices even in non-working condition.
Some later devices are also surprisingly difficult to obtain, or are very expensive. I had some trouble obtaining some eight pin 7-series valves for a set made in 1949 - I needed five, and didn't fancy paying twelve pounds each for them!
Some dealers offer a valve testing service. Normally this only consists of testing the essential characteristics to keep the price down, but this is generally sufficient to tell you if a valve is serviceable or junk. However by the time you have posted the valves to the dealer, paid for testing and return postage, it would probably be more cost-effective and quicker to just order some new ones.
In practice valves do not fail as frequently as might be expected. I have repaired sets that have been in daily use for 40 years of more, and which still have the original valves in good order! This is particularly the case with older octal-based valves etc. Some newer valves (notably the UL41) fail with monotonous regularity - and this particular example is quite expensive too (twelve pounds typically).
Another exception is the small 7-pin battery valves (DL91, DAF96 etc.) which do not seem to be especially long-lived - particularly in mains-battery sets. Fortunately these are cheap (three pounds each from Valve and Tube Supplies at the time of writing) and most sets only use four, so it makes sense to replace them all.
Boxes of Unknown Valves
If you are advertising for valve radios in the local paper, you will inevitably receive calls from people offering boxes of old valves. You will also find similar boxes in vintage radio auctions etc. This may sound wonderful, but from my experience, they may be of limited use.
There will often be many television types (PCL82, PY88 etc.), that are no use for valve radio repairs! If there are any radio types, I suggest you pay no more than ten pence per valve, as the majority will be in a poor state (no matter what the seller claims).
I bought a tray containing a couple of hundred valves in an auction a while ago for £5. About a quarter of them were radio valves and over half of those tested OK. The rest were put into another auction a few months later and sold for £2. So I got about 30 useful valves for £3 - I couldn't complain!
It was the practice of many engineers to put the old back in its new counterparts box as an emergency spare, so although you may be offered apparently "new valves in original boxes", always be aware of this ploy. Some sellers emphasise that the valves are in boxes. That is irrelevant. What is important is the condition of the valve. A dead valve in a box is worth no more than a dead valve without a box!
Other Components and Parts
As mentioned in the Buying Sets page, it is always worth buying scrap sets and chassis as a source of components. Obviously it helps to be a bit selective and to choose scrap that is similar to the sets you collect if you are to make the best use of the parts. Scrap sets and chassis sell at swapmeets for next-to-nothing (typically under a fiver for a set and a pound or two for a chassis). Look under the tables at these events, that's where the boxes of junk tend to be. If they are still there late in the day you can probably buy a box full for a pound or two because the vendor doesn't want to take it all home again.
Replacement of resistors and capacitors is dealt with on the relevant pages on this website, output transformers are dealt with on the Output Stages page and mains transformers on the Power Supplies page.
Generally the main problem areas are items of cabinet trim, speaker fabric, tuning scales and knobs. Sid Chaplin supplies a selection of modern replacement fabrics as well as rexine, brass clips and hinges, handles etc. - details on the this page. Otherwise you will have to resort to searching the boxes of knobs at swapmeets, finding similar parts on scrap sets or making something up to do the job.
Unless the work required is minimal, it is worth obtaining a copy of the relevant Service Sheet.
I am selling three Vintage Radio Service Data CD-ROMs which will probably be of interest to those who repair sets regularly. For more occasional or one-off jobs I also offer a service data by email service. There are also a number of suppliers of photocopied service data - see the suppliers page.
If you have copies of the Newness Radio and Television Servicing books you can access a full manufacturer/model number index of the radio service data included at this page on Allan's Virtual Radio Museum website.
Nigel Hughes adds the following useful suggestion:
Radios were generally restyled from year to year to appear as new models at the Radio Show etc. However, in many cases, the changes in the circuitry from year to year were less drastic. Philips were using the same RF section to their superhets from 1934 to 1936 at least.
I have recently been restoring a very rare DC mains Pye of circa 1933, which may have been designed specifically for use on board Cunard Liners. This radio is not on the CD-ROM and I can find no trace of a service sheet anywhere. However, it bears similarity to the Pye E/B, which uses the same chassis hardware, coils and wavechange switching. I found even this limited data to be of enormous value in circuit tracing.
Similarly, I am currently working on a 1960s Dynatron transistorised FM/AM radiogram which I bought for the excellent Goldring Lenco Transcription deck it incorporates. Although the specific model is not on the CD-ROM, there is a battery portable AM/FM radio which uses the same RF and IF modules and this has been a great help.
The advice is that, if you cannot find the specific service sheet you want, it is always worth looking at the contemporary models from the same manufacturer, as you will probably find that parts of the circuit are the same.
I can confirm Nigel's suggestion. In the 1950s, many manufacturers used largely the same circuits in number of models, often over several years, with only minor changes to suit the features of the particular model (such as tone controls). Bush in particular did this, as did Ekco and many others. The Thorn group made radios under the HMV, Marconiphone and Ferguson brands, and later added Ultra. They also made sets for Philco. All used the same basic chassis and circuits. So if you cannot find the exact model, look for information on that manufacturers models from around the same period and having the same basic features (such as the same wavebands and AC/DC or AC only).
A valve data book is useful, particularly if you do not have the service sheet for a set. The publishers of Radio Bygones are offering reprints of Bernard's valve data books for very reasonable prices. It is also worth looking out for original data books - particularly Mullard publications.
I am also selling a Valve Data CD-ROM which contains scanned copies of a number of useful publications, including Mullard, Mazda and Brimar valve data books and the aforementioned Bernards books.
There are also a few websites offering valve data on-line. Probably the best is Duncan Amplification, which also has a useful database program (giving basic characteristics and pinouts) available for free download (Microsoft Windows only).
Transistors and Transistor Data
Replacement transistors are actually more difficult to obtain than replacement valves. Details about the different ranges, problems and replacements are given on this page.
Some transistor data is contained on the Valve Data CD-ROM.