I can't help with the gems, but you may want to check local craft/art supplies stores. They usually have a section for bags filled with various "gems" and stones. Maybe you can find some there.
As for the "gold parts," I'd go with craft foam. You know, those cheap sheets of colored thin foam found in places like Michaels and such? It'd probably be easier to work with, and although it may not look as good as plastic or something, it's definitely cheaper and easier to find.
You can probably make a template out of newspaper which fits you, trace it onto the craft foam sheet, cut it out, and paint it. I don't know whether craft foam needs to be primed or not, but you may want to test out painting on a small section of scrap.
You may also need some sort of a sealer as well to make sure the paint doesn't chip, but if this is going to be a "wear temporarily, not aiming for costume contest" you can probably skip the sealer.
Good luck with the costume!