It depends how flexible you are and what you want. If you want acrylic, i think fishtanksdirect has the best prices on large tanks and are a pretty well regarded company.
If you want glass, i think glasscages has the best prices. The thing is, you have to go meet them somewhere to pick up the tank (which may not be close to you) or else pay pretty dearly for shipping to your door.
If i was ordering a big tank, i would try out customaquariums.com. ive heard a lot if good things about them. Im leaning towards glass over acrylic these days as well.
Whatever you do, DO NOT order from "american aquariums". I had a really bad experience with them that was rife with incompetence and poor customer service.