Digikey and Mouser, if you want guaranteed good parts. They have a huge selection and they both accept small orders. Digikey will add sales tax wherever you live, but Mouser does not for most states. Both accept orders online, and have excellent online search engines.Data sheets are online for everything they’ve got.
If you open an account (free) you can check your order status and order history and re-order off past orders. The cheapest shipping option for both of them is US Postal Service. Digikey is better at shipping your order out the same day – their shipping cutoff is late, 7pm Central or later, but Mouser’s USPS cutoff is 3pm Central and they’ve missed it with some of my orders, even some that were placed hours earlier. So if you’re in a hurry, pay the sales tax and order from Digikey. There’s very little between the two for prices, but always check them both.

Newark has stuff that the above two don’t. It’s affiliated with Farnell, a European firm, and can get you anything in the Farnell catalog, including a lot of European parts not otherwise known in the US. For some reason I always forget about Newark, probably because I don’t have an account with them, but I do order from them from time to time.

For cheap, go to Ebay and buy from Hong Kong. That’s where I get my LEDs in bulk. They’ve turned out to be fairly reliable, except for some UV LEDs where every second one failed after an hour or so, but hey, I only paid 11 cents each for them.