Watch salvation army and your other local thrift stores for tanks and enclosures- they turn up pretty regularly and you can find some great bargains- just don't go much over $1 a gallon for enclosures unless they include a strong lid, lights or a working heat mat.
Craigslist is also a good resource in many areas.

Be ready to clean and disinfect anything used- take that into consideration when you decide what you are willing to pay for an enclosure. Sometimes a tank isn't worth it if its heavily crud-encrusted. (Harsh cleaning chemicals can be very bad for pets.)