Aurarum - Great supplier of parts for upgrades and DIY printers, also make their own filament.
International
Hobby King ( RC stuff, batteries, motors, servos , etc are all great for using in ROBOTS and other stuff as well )
Deal Extreme (DX) is really cheap with free shipping, you can get servos, LEDs, chargers, batteries, lenses, lasers, other hobby parts, and so many other random items there - most of questionable quality and accuracy.
Banggood Similar to DX, slightly cheaper and more questionable
AliExpress Cheaper still, even more questionable, but a very wide range