For Revell and Lindberg-- Evers Toy Store (www.everstoystore.com)...bought dozens of kits from them for over a decade. Great customer service.
For AMT (Round2, Stevens,recent OOP) and Revell, Model Express (www.modelexpress.net).
For import, Hobby Link Japan (www.hlj.com). Been buying from them since about 2000.
These are 3 I use the most, there are others like Hobbylinc that I've used occasionally.
And of course, eBay for old kits, and occasionally newer kits I can't find elsewhere.