I lurk Craigslist and the Facebook marketplace a LOT for pet supplies and live animals. Very nice finds if you keep a sharp eye out! Outside of those, you could go thrift shopping or post on your regular Facebook feed. One of your friends may have something lying around that they'd never think of otherwise! I just got a nice 55 gal for free from a friend after posting that I was looking for one. You never know!

Just make sure to sanitize whatever you find!

Good luck!!