I saw a 200gallon square tank before, and it was really cheap (around $300 for the tank, stand and canopy) but it was also fairly bulky and probably not very good as far as decorating is concerned. Don't remember the dimensions, but I'm sure there's a gallons calculator out there somewhere.
I'm pretty sure it was made by All Glass Aquariums. Call them and find out if they have a distributor that they can ship to, or if they'll ship it directly to you.
[ September 19, 2001: Message edited by: davelin315 ]
My LFS has 2 cube tanks on display. They each hold about 140 gallons and has a center overflow. They are viewed from all 4 sides. The are nice tank, especially if you plan on viewing every side. The dimensions are 36x36x25.
One of the most interesting tanks I ever saw was a cube tank approximately 2' square that was setup like a tidepool. It was positioned down low and used pendant lighting so that it could be viewed from the top as well as the front with the live rock extending up out of the water so it had an intertidal zone. The rock was arranged along three sides so that from the front it looked like you were looking into the side of a small tidepool. It was in a pet store many years ago and I thought it would be an interesting project to try to set something up like that. Perhaps even a cold water version with local tidepool specimens.
As far as finding a tank, most LFS have connections to have an acrylic tank made to any desired size.
I've been "documenting" the setup of my cube tank w/ pictures, but haven't put anything down in writing yet. I'll post some pictures when the tank gets to be a little "prettier" as no one likes to look at a brown tank...