Here are some suppliers I’ve used, and liked:
EZ Robot – Lots of great ways to get started in robotics, or expand on a robot. Not only has a great controller, but tons of very innovative software to get you up and running quickly, and lots of videos and tutorials. Highly recommended!
Pololu – Really good Motor Controllers (TReX), Servo Controllers, great source for Sensors, high efficiency power regulators, and lots of other stuff.
SparkFun Electronics – Arduino, GPS, WiFi, ZigBee, Compass, and lots of other sensors and great stuff! My only complaint is they have so much stuff it’s hard to find things on their web site!
Trossen Robotics – My favorite supplier for Dynamixel Servos and brackets. Lots of Robotis stuff including their “Bioloid” robot kits and Arduino. Check out their humanoid kits and Hexapod videos – cool!
Parallax – Originally famous for the Basic Stamp, they have lots of good stuff for the robot experimenter. I find them a little more expensive, but they seem to do a good job of making everything easy to use. They have a good selection of sensors, and check out their new “MadeUSA” and “Eddie” bases, and their “Motor Mount and Wheel Kit with Position Controller”.
Robot Marketplace – VEX Robots, Surveyor Robot, Gears, Small to Mid-scale Combat Robot parts and motors, etc. Lots of stuff that’s hard to find anywhere else. This is where I got the chain and sprockets for Loki’s Shoulders. Big selection of various size geared motors. Also, a great selection of Robot Books!
Devantech – AKA Robot Electronics. Makers of some of the best sensors around! Their sensors are available at many suppliers, so you can either buy from their website (if in Europe) or Google for the part you want. I have used their SRF04/SRF05 Ultrasonic sensors, Compasses, and Thermopile array.
DigiKey – If you want electronic components, you can get them here. Not always the cheapest place, but they have everything. This is setup for Electrical Engineers, so you need to know exactly what you are looking for, including the package style. Sometimes project pages will give you the link to the exact component on DigiKey’s website; that helps a lot.
Acroname – Really nice, high-end stuff, if you can afford it.
CrustCrawler – Dynamixel Servos and metal servo brackets, and a variety of metal hexapod robots.
Lynxmotion – Hexapods, motor controllers, etc.
Portland Robotics – Great prices on stuff, and some things I can’t seem to find anywhere else, but I’ve heard that sometimes people have to wait a while to get their stuff.
www.solarbotics.com – misc stuff, including wearable electronics.
www.electricscooterparts.com/specialtysprockets.html – Chains, Gears, Sprockets!