The screws should be #2, but I'm not sure which thread, usually something coarse. (So perhaps #2-24 or #2-28; don't know.) Unfortunately, these screws can't be easily found in hardware stores and may have to be ordered on eBay or Amazon. You may want to measure the length of the screws you already have, count the threads, and deduce the threading from that. Of everything, length matters the least since they can be cut down.
Or do the thrifting thing, but that's expensive if you just want only screws. The eBay option should cost no more than a few bucks with shipping included, and you'll get at least 50 or 100 or so screws. Problem is getting the right thread, but thrifted blasters can have the same problem.