Like Mike says, Home Depot usually has a selection things like this. I find them in rotating bins with drawers at the end of the aisle with the nuts, bolts screws etc.
You might be looking for the male piece of the snap though.
Did the screw break off or did the snap piece break?
Quality varies greatly with fittings like these. I'd look for a stainless steel replacement part. A marine hardware store would have them.
This photo shows two:
The lower one is much better quality than the upper one. It is stainless steel so it won't rust. The upper one is chrome plated steel and would be more suited for indoor use.