I bought this kit nearly 10 years ago and built about 8 cars as well as doing other odds & sods with it in that time;-
https://uk.rs-online...t-kits/6235549/
While the kit only goes to 6 mm, 8 mm mandrels are available and I think stainless Rivnuts too, though as Nick and Sonikk4 have mentioned, Stainless while seeming a good move is actually a really bad move. It make adjacent metals corrode or rust, when used with stainless fasteners, it galls up and as a fastener, it has very very poor strength.
I just use the Zinc Plated Rivnuts, been super pleased with them.
I'd be lost big time without this tool these days, love it.