I put up a wanted add on craigslist looking for glass. Ended up getting 5 sheets of 30"x60" 1/4 glass for free! They owned a building that once had a glass laminate company. That company was evicted and left a bunch of glass sheets. I was basically clear out the building for them. Granted, I got lucky here, but you may find someone putting new windows into their house that would be willing to give you the old ones. You can then take whatever glass you get to a local glass shop and get it cut for a few bucks.
Now, I don't know where you're located, but if you (or anyone else) is around Statesboro, GA, I have 2 of these glass sheets left over and would be willing to pass them on to another beardie owner for free.