RealOEM says they're M6 18mm long. IIRC they're high tensile with a black coating to resist rust. I'd just pull them off a spare car, maybe a local salvage yard? I'd figure every BMW would use similar bolts.
I'll bet any Pep Boys, Advance Auto, etc would have them in the fasteners section, though mine is often out of stock and they're pretty low quality. Dealer will have them too, likely at three times the cost. Take one along to see if it matches up.