Take the nut to a hardware store, and they should be able to find and give you the appropriate bolt. Ace Hardware?

I have a nice mom-and-pop neighborhood hardware store nearby. I recently took the broken, rusted nut from my brake caliper, and the generous owner took a minute of his time to help me find the appropriate thread and size nut and bolt. They sell nuts and bolts loose. I think I spent about a buck, or a buck and a half, for both the bolt and nut. Well, the bolt was a tad longer than the stock one.