Find a local builder - aquarium society meetings are a great place to meet useful people.
Or build your own. Lots of good information on the web about making any size of tank, and then you only need to call your local glass shop and pay materials and maybe delivery. Silicone is great stuff and very hard to make a mistake.
For a BIG tank, look into plywood as an option. My 240 cost me $300 delivered. Materials for my next one (building it myself) will be under $200.
Hope that helps a bit,
/Kris