There's a site boltdepot.com that i get all my bolts for when doing this sorta thing
Also I agree you're better off avoiding the stainless for aluminum cases, but in reality it probably doesnt matter. I dunno, theyre a bad fit from the standpoint of galvanic corrosion but the aluminums such a huge surface area it likely doesnt matter, it's mostly just how i convince myself it's "OK" or even "better" to buy the marginally cheaper Zn plated or black oxide
but yea check out boltdepot.com, scroll down for the metric section: individual bolts with no minimum order, large variety of sizes and prices under 25-50cents a piece, even for the 316. Its a good move, they've been a solid goto vs mcmaster which can be pricey and kinda gouge you on shipping, though it is nearly instantaneous... (edited)