Those are all solid sources. I'd like to add trossenrobotics.com as a source for Dynamixel servos and parts, as well as walker and arm kits, and servocity.com and robotshop.com for more generally robot-ey things. And adafruit.com for more Arduino-like and project-like things.
My ranking of how much I buy from those sites is probably:
1) digikey.com -- best price and selection if you need raw electronic parts, with good customer service
2) pololu.com -- very well tested gear with great customer service
3) trossenrobotics.com -- dynamixel based things, with good customer service
4) jameco.com (which I buy from when I need something today, as they're just up the street from me!)
5) sparkfun.com -- when the others don't have what I need
6) adafruit.com -- when the others don't have what I need, with great customer service
7) mouser.com -- when something's out of stock at digikey.com
There's also mcmaster.com, hobbypartz.com (servos, batteries, RC,) hobbyking.com (servos, batteries, RC,) amainhobbies.com (RC,) integy.com (RC,) element14.com, and a bunch of other companies that may or may not come in handy depending on what you want to do :-)