You can often pick up used glass tanks at yard sales or through craigslist or that type of thing. Sometimes larger fish specialty stores will sell off used glass tanks at very reasonable prices because of some small defect that makes them unable to hold the pressure of water but is fine for reptile use.

Heck I just put the word out to a few people and had used tanks they had sitting in their garages and basements given to me. Just had a huge 50 gallon one dropped off yesterday in fact.