I guess what country you stay in limits the choice of suppliers. We are still quite fortunate in the UK in that we have several large store retailers and one line ones also. The real trick is getting a good grade stainless steel screw of roughly the correct dimensions. The other tip is to get some small rubber o rings and place one under the head of the new screw, a little dab of silicon grease before fitting the screw. This will both lubricate the thread and greatly slow down water ingress.
I sourced my screws from Halfords.