Excellent ideas and great creativity on the sites mentioned! Just a couple more ideas:
1. Try your thrift stores and look behind grocery and department stores. They sometimes get rid of broken display cases that could be re-purposed, just be sure to ask before you snag anything
.
2. Got any old screens laying around? Stapled to scraps, you can make lids and windows, just don't use the nylon screening under the heat lamp.
3. New construction sites often have "ends" and scraps they would be happy for you to carry away for them, but of course, ask first.
4. Wooden pallets often have nice planks of wood if you're willing to demo and de-staple 'em. Our lumber yard usually has a pile with a "free" sign for scraps...
My friend just moved and had an old dresser (36"H x 28"Wx16"D) that was headed for the landfill. I turned it on it's side and used parts of the broken drawers to cover what was the bottom and filled in the top and bottom drawer sections. I made a door covered in hardware cloth for the center and cut a hole in what was the side (now the top) for the heat lamp. I added a small loft (15 x 15) about a foot off the floor on one side and made a little ladder out of scraps. It ain't pretty, but they have lots of room for now, and it was mostly free with the scraps I had on hand.
Good Luck!