I'm in the process of gathering information on this very thing. I have found that you have a lot of options when building your own large tank. I've came across plans/info for constructing them from poured concrete molds, cender blocks, all glass, acrylic, fiberglass, and marine plywood. If you'd like some of this info, email me.
I intend to have my tank up and going by the end of summer. I plan to build a 2000-2500 gallon aquarium, built just for sharks. Real sharks