Jameco, jameco.com
JDR Microdevices, jdr.com
McMaster Carr, mcmaster.com
You can also buy from the Hong Kong Ebayers. They always have amazing deals on components but can take weeks to get here. I picked up 12 pushbuttons for $10 shipped (little guys, not the arcade ones) in the specific style I was looking for. I also bought 50 white LEDs for like $7 with free resistors at the value of my specification!