Where can I find random pieces of the Leggo Mindstorm kits?

I have been messing around with Leggo Mindstorm robotics pieces at school for a while. I build from my own designs. Is there any place I can pick up just a grab bag of random pieces (sticks, joints, snaps, motors)? I don't want to get a whole kit when I'm not going to build that specific robot.

Lego sells all the different mindstorms parts separately, along with power functions pieces and bricks.