If you are building a full-size R2-D2 astrodroid from wood or plastic, one of the better options for motors are the MaxDrives.  These drives can use either old-school brushed CIM motors, or much more modern NEO brushless motors.  But the output shaft on the NEO is about a quarter-inch too long, so it needs a spacer.

If you don't know what I'm talking about, you probably don't want this.

This is a very simple thing to work up, but having done it myself I figured I'd save other folks the bother and upload it here.

If you are interested in building your own R2-D2 or other astrodroid, go join Astromech.net!  Be prepared to do a lot of reading, though.  It's a complex project that usually takes folks a few years to finish.