Try calling Dave at Dave's Rare Fish. He often has some old tanks cheap. Also, talk to LFS. When they have broken tanks they throw them out. A cracked tank can usually be made useable with a simple overlay of glass plus silicone. Might also look at alternatives other than glass/acrylic. Plastic tubs should work fine. Another thought is that a 55, for instance, can be sectioned off into several tanks with cheap glass (be sure you seal them well and fill them equally so it doesn't blow out).
Bill
215g FOWLR... and anemones, GSP, gorgonians... carp, that isn't FO!
"I killed my first SW Fish in 1971..."