The production of prototypes is less formal than a full design but can provide provide answers that reduce risk and guide detailed design. Aside from current knowledge hobby related websites are an excellent source of ideas, techniques, and materials. This list is not exhaustive, but it does include many sites my students have used to purchase materials and find help.
Good General Sites:
Materials and Components:
Rapid Prototyping - Solids:
Rapid Prototyping - Circuit Modules:
Rapid Prototyping - PCBs:
Solid Model Libraries (STL and Object):