A good website and good customer service goes a long way...that's why my favorite vendor for new Revell and AMT kits for a long time has been Evers Toy Store (www.everstoystore.com).. I've bought from them for years, they have pics, an easy to use shopping cart, etc...and they are fast to ship in stock items, do preorders effectively, and use UPS, which I prefer over DHL... The only negative with them to me is that they don't stock the Model King special issue kits (but I have other sources for those kits).
I've also used Model Express w/ no problems. I've looked into buying from Model Roundup, Fantastic Plastics, and Model Empire, but their websites don't work very well (no shopping cart, too much typing needed). Tower Hobbies has a pretty nice website and selection, but I haven't tried them yet.