http://www.andymark.biz/
http://www.howstuffworks.com/
http://www.firstcadlibrary.com/
I am not sure what exactly you are looking for, it would be helpful if you were specific. FIRST cad library has drawings of a lot of parts given out by FIRST. Andy-Mark site sales parts (transmission, wheels). You can also go to how stuff works and search for anything you are looking for mechanically.