There are some brands which make pistons for RD. Prox, Wiseco, Mitaka. I suggest going for Prox, it is recommended by many RD enthusiasts. Try searching in eBay, Yambits for these pistons. yambits.co.uk/rd350-piston-kit-025mm-p-8148.html?osCsid=1b74b2oa8q2jau87kdsdiqjek5
