This is a list of suppliers that I have used or believe to be a good option. Its for my reference as much as it is for you:) I am mainly concerned with places that would have things related to Arduino, raspberry pi, robotics and general electronics. Price is certainly an issue as well as reliability and selection. If there is really someone I need to look at, send me a note at: info (at) robo-t.org
Newark.com Large inventory. Geared for professionals rather than hobbyists. Very happy with their service.
Yourduino.com I made a sizable order from them. Their prices are very inexpensive since they ship from China. I was happy. Will be using them again. They have useful information on their site for hobbyists.Their wiki. Terry is helpful:)
Adafruit.com I haven't ordered from them yet, but apparently are reputable and worth a look. They have a lot of useful information on their site (learn.adafruit.com) They are also design kits and such.
NKCelectronics.com I haven't ordered from them yet, but looks all right. They also sell through Amazon.
Radioshack.com Who hasn't bought from them.