I have the same problem so I thought I'd post to help others find the o-ring kit. First I am not a plumber but a doe-it-yourselfer so take my feedback with that in mind.
Problem: Dripping Grohe 08 355 Shower Faucet. I went to http://www.grohecatalog.com/line/Grohmix/Shower%7CSafety-Valves/...
