Hiya Shutterpuppy, and welcome

I don't know just how small that screw is that you're missing, but I got one for my SLR camera back from a local optometrist which cost me all of nothing. They just went out the back, had a look through their spare screw supplies and found one that matched in size. When I offered to pay, they said not to worry about it. Basically, any place which sells and/or repairs eye glasses should be able to supply you with a screw tiny enough to fit your needs.