Here's my suggestion, although Dan might disagree. Buy Spartech polycast or Cyro GP 3/8". If both of you are going to build tanks, you could split a sheet. Cut it with a 60 tooth triple chip grind carbide blade, or pay Dan to cut it for you, if he'll be nice enough to do it. You can use weldon #4, do not use weldon #3, to solvent weld. But, a far stronger joint would be with weldon #40, which is 2 part polimerizing cement. Unfortunately, that complicates your building a bit, because you have to rout shallow grooves, like "dado" grooves, to set the edge of the piece you're cementing in. At the least, if you solvent weld, you should leave a 1/4" overhang, then flushtrim with a router. Actually, if you can get your hands on a solvent called "M.C. bond" it's stronger than the weldon solvents.

In terms of saving money, it is cheaper to buy acrylic than a finished tank, but you could probably get Dan to make you a nice one at a reasonable price. The first sump/refugium that I made worked out okay for the few months that I had that set up. The test, though, is whether or not it will last years. Certainly the cheapest route is to buy a used glass aquarium that meets your needs. Cast 3/8" acrylic, Spartech or Cyro, is about $250 for a sheet at least. (I paid more than that for black.)

Of the local places, I think the service at Plastic Supply of San Antonio is the best. They do the best cutting IME. Good luck!