One of the things I like about VirtualCAD is that they have Fastenal part numbers. Since we buy a lot of screws from Fastenal, it is convenient to use the model from VirtualCAD and then we already know what part to buy.
http://www.virtualcad.com
You have to register to download CAD models. I usually download the iges version and then open it up in inventor and save it as an inventor part. This avoids the occasional inventor crash that seems to happen every now and then when I am mixing model types in my assemblies.