From what I've heard, the ATE pistons are not available, Motobins, a BMW parts supplier in the UK, has stainless pistons, I think the last time I looked, they were about $75US for a set of two pistons, enough to do one caliper .
I was a bit surprised, not to find any build-up of 'grunge' anywhere in the brake system, when I first broke the fittings loose, I got clean fluid, not what I was expecting .