I have fitted stainless pistons, but that was ten years ago and I can't remember where I got them from.
However, I wasn't too impressed with the seals that came with the kit. They looked like someone had chopped up a rubber tube with a Stanley knife. Accordingly, I bought some OEM seals separately, and they actually looked like they were made for the job. The OEM seal retaining rings are much easier to press into place without bending them, as happens with the cheap generic ones.