Check out the ads in the back of Practical Classics magazine, there is a company that sleeves the wheel cylinders and provides new stainless pistons so long as you have parts to work from. I can't remember their name but it would be worth a phone call to check it out. I have got around this problem in the past by using the seals and pistons from a drum braked VW beetle wheel cylinder which are the same internal diameter and just cleaning out the cylinder as best as possible with some very fine wet and dry. This bodge was only intended to get me the 150 miles back home but ended up lasting a few MOTs without any problems.

The only spares I have are almost certainly as wrecked as the ones you are replacing, in fact I have no idea why I have bothered keeping them !!!

Regards - Alastair.