You can look locally, but I doubt you'll find 6-48 screws anywhere but at a gunsmith. I gave you two easy sources, what the heck do you want?

They both know exactly what screws come with the WGRS-HR sight, so give em a call!
Woodchukhnter should have sent the screws that came with the sight if he's a great guy, I've sold two of em myself and sent the screws with both of em, I've also bought one used from Raynor, he sent me the screws with the sight.

You could also order them from Brownell's, a dozen of the short ones, and a dozen of the long ones, they only sell em by the dozen.

Tim