The Bolt Depot carries a nice selection of of stainless and regular hard steel (grade 8.8) metric fasteners with secure internet ordering.

I find I'm having to restock my metric collection after years of work on the MBs I have nurtured. These are not the right substitute for high stress applications (e.g. brake rotors, con rods) where you should use OEM, but they are great for non-critical tasks. My local hardware store always seems to be out of the ones I need....