My personal choices.
For parts:
Jameco www.jameco.com/
My favorite! I buy from this guys since 1977 or so (used to be James Electronics). Prime components, very low prices, excellent packing (IC's in their special container tubes in antistatic bags), affordable shipping cost & fast delivery. I highly recommend. You also get their great printed catalog free with your first order. Check it out!
For Boards, shields, DIY kits, etc:
Sparkfun, AdaFruit, Electronics123 & eBay.