Along with what MinnFlyer says, I like these also:

Quantum models, http://www2.mailordercentral.com/quantummodels/
Good selections & prices. Free shipping over $150

The realm, http://www.hobbiesandgames.net/
Good prices, ok selection. Free shipping over $100

Balsa products, http://www.balsapr.com/
Good prices on accessories (.ie, glue, epoxy, servos, etc.)

Radical RC, http://radicalrc.secure-mall.com
Good prices on Thunder Tiger engines and other accessories