yea the largest I've sseen any of my lfs carry in stock is about a 180 sometimes they'll have a 225 up for sale. but anything bigger, they can order it, but then again so could I and for cheaper. Most are generally custom made or homemade.
When I get my large tank, I'll be getting mine (hopefully) from www.glasscages.com
got mine from someone that had it made, then he wasn't really into the hobby anymore so in his eyes, it was takin up space. in my eyes, i need the space. someone whos tryin to make space = cheap. search n you might find one. or custom order one but any tank above 300gal custom made is basically 1500+ depending on the dimensions you go with.
Try this place: http://www.tropicalfishstore.com/
The company is called ARB Distributors (Missouri) and they deal mainly with Tenecor Acrylic tanks (based in Phoenix, AZ).
I just got my 300G acrylic about a week and half ago (72x30x36). It's cycling right now. They also make custom stands...probably, the best and most beautiful that I've ever seen commercially. Be prepared to pay out the @$$ though because custom tanks that size are not cheap unless you make it yourself. I did a pretty good search on the web and ARB Distributors came out on top as far as price. The tank included everything: wet/dry, 2 Iwaki pumps, 3 Coral Life Aqua Plus Freshwater Compact lighting systems, and custom made canopy and stand to my specs. The w/d was the easiest filtration system of that type that I've ever put together. All I needed were the heater, miscellaneous accessories and fish, but I had the tank set-up and running that night straight from delivery.
I agree w/ vyperboy! TENECOR mfgrs. some excellent set-ups http://www.tenecor.com/ . My 125gl came from them & I'm extremely satisfied with the quality & craftsmanship. If I ever get enough room for a 300/500gl, that's where it will come from. As for now I'll just have to enjoy my lowly 125gl.