48 inches long, 24 inches high, 18 inches wide. It is the most common in a more 'square' 90 that I have seen widely used.
There are many ways to do it but this is probably the most efficient for space reasons, and building material cost whether you choose glass or accrylic. take the dimensions, multiply all, then divide by 231 (the formulae for how many cubic inches in a gallon ) these dimensions will give you a 89.76 gallon tank.
In that case, you could always just get a 125 like the one I'm working on. Call around to all the glass stores in your area and see what kind of deal they could cut you on some remnant glass pieces they have laying around.
A 90 or a 125 would need to be at leas 3/8 thick material though. dimensions for what you're talking (for swimming room back and forth)are 72 by 22 high X 18 wide for a 125. do a little less on the length by about a foot and you should be close to a 90.
OOps !! just saw that you said you wont be making it yourself. you should try the glass places anyway though. could save you a couple hundred bucks maybe just to get the material and have someone else seal it with silicone and make the necessary stand and braces, etc.