I have had great experiences with these suppliers and I would recommend you check them first. They should also be your first stop for other electronics such as servos or sensors. If they don't have what you are looking for you can look through the lists of electronics suppliers on the Society of Robots site as well as the Arduino site.
If you STILL can't find what you want try searching on google for specialty sites, you just might find what you are looking for. Remember to always check multiple sites to find the best prices! All this should give you a good idea of how to locate and purchase electronics for your projects.