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).