I like Evers Toy Store for new AMT, Revell-Monogram, and Revell of Germany kits. I've bought from them for 20 years or so..they have a easy-to-use website, w/ pictures of all the kits, and do pre-orders. Good customer service and quick shipping.
www.everstoystore.com
I also like Model Express, I've bought new and older (mostly '80s and up) kits from them.
Model Express
For Fujimi, Tamiya, and other Japanese brands, I buy from Hobby Link Japan.
Hobby Link Japan
For Model King special issue kits, I've lately used a couple of reputable ebay dealers.