If it's the one I'm thinking of, you are looking for a dome-head 4mm hex, M5 x 8mm length or something similar (measure the one from the other side). If you have a specialty hardware store or fastener supply house, they'll likely have it - if they're really good, they'll have both stainless steel and other versions. A regular hardware store may not have the hex dome head machine screw, but would certainly have Phillips dome head and hex flat head metric screws of the right size, washers if needed, to get you by for now.
If you can't find it in rural Kansas, the shops on OKC and KC should have the correct one.
