They sell stainless steel metric allen wrench bolts at my local Ace hardware store. I have a stash in my shop. It's good to have several lengths. You want one that uses all the threads in the hole, but which doesn't protrude out the end far enough to cause any problems. I also replace all my rustable steel bolts with stainless.

It would be nice if they included the perfect hardware with each accessory you buy, but it's so easy and cheap to have your own stash I don't worry about it. It makes me feel prepared. (I have a stash of tubes, cables, cable housings, ferrules, and those little things you crimp on the ends of cables too. I'm almost like my own bike shop!)