If you have a watch but no key then usually the easiest thing to do is to buy a complete set of 14 modern keys which can usually be found for less than £6 by searching eBay or on Google etc. Individual new keys are available but the price difference is small so it is probably safest to buy a set and be sure of getting the correct size.
As a general rule antique keys are easier to use as normally there is a swivel between the ring and the body of the key which makes it generally easier and if on one end of a double albert it avoids twisting the chain as you wind. You may also be able to find an old tube like key which is adjustable for different arbor sizes.
If purchasing an old key you will find that relatively plain keys will normally be cheaper than those with advertising or a makers name on them.
"Spider" keys are also available with a number of keys radiating off of a central hub but these are expensive and really only of benefit if you have a large number of watches, or restore them.
There are a few things to be aware of:
1. Old keys tend to stretch and wear with use so if your existing old key was not original to the watch then the equivalent modern watch key may be a bit small and buying an old key without being able to test it can be a problem as it may be too big.
2. The winding and setting arbours on the watch should normally be the same size but may not be if one has been replaced as part of a repair or one is very worn, it is however sometimes possible to use a compromise size.
3. The modern keys have a bevelled entry into the key, this makes it easier to locate the key on the arbor BUT I have found that frequently this prevents the key from properly engaging when trying to set the time on a front set watch due to the setting arbor being to short above the hands if this happens it is necessary to file or grind the bevel off of the key which is quite straight forward as the metal is fairly soft.
Watch Key Conversion:
Watch Key Size:
UK: 00 = 0.95mm.....Indian: 00 = 2.00mm
UK: 0 = 1.00mm.......Indian: 0 = 1.90mm
UK: 1 = 1.05mm.......Indian: 1 = 1.80mm
UK: 2 = 1.15mm.......Indian: 2 = 1.75mm
UK: 3 = 1.20mm.......Indian: 3 = 1.65mm
UK: 4 = 1.30mm.......Indian: 4 = 1.60mm
UK: 5 = 1.40mm.......Indian: 5 = 1.50mm
UK: 6 = 1.50mm.......Indian: 6 = 1.40mm
UK: 7 = 1.60mm.......Indian: 7 = 1.30mm
UK: 8 = 1.65mm.......Indian: 8 = 1.20mm
UK: 9 = 1.75mm.......Indian: 9 = 1.15mm
UK: 10 = 1.80mm.....Indian: 10 = 1.05mm
UK: 11 = 1.90mm.....Indian: 11 = 1.00mm
UK: 12 = 2.00mm.....Indian: 12 = 0.95mm