They carry a wide range of products. I came across them in the search for organizing drawers and cabinets for my kitchen. No exactly stagecraft related, other than we often have a variety of odd objects that need to be stored, and special hardware that needs to be found for specific scenery purposes. And this place is a nice blend of both.
Deerwood fasteners are a good resource for square head screws. I have mixed thoughts about square head screws. But, some will swear by them. Nevertheless Deerwood also has a fastener guide located at:
And just for fun:
Check that out for some fun. Clever idea, fun, and useful - what more can you ask for in product design.