McMaster-Carr catalog - McMaster-Carr has models of a lot of their fasteners. They aren't all perfect, and some take a little rebuilding, but it's a lot better than pulling out the Machinery's Handbook and starting from scratch. Find the part you want, then once you get to the part page (with the little drawing of the part), at the top there is a download button. Select IGES and it opens right up in Rhino! –Sam Page
VirtualCAD - Free manufacturer part number interchanges in tandem with free high-detail 3D CAD models of mechanical components.