Your local craigs list, yard sales are a couple of good place to get cheap tanks.
I don't pay more than $1/gal on tank only, if they have equipment with them like pumps, filters, heaters, lights/hoods etc.....I won't pay that much for them all depending on what they look like, how old they are, how long used...etc.....
When I buy used, even when they say "no leaks" which may be true, but after they sit dry, or the move in general from being twisted or moved with water..... I do a leak test anyway and check seals.
I always clean them with a diluted bleach and let them sit full of water for 24 hours or longer to make leak test and with the diluted bleach in the full tank I will set up and run the filter on it too, one, to clean it with the bleach and to check to see if it works, to listen to it...etc.....If I got a used heater I like to do the same thing to check if it works and to clean.
I use razor blades to scrape off lime scale, dried algae and the crud in general.
Hood and light, I check and I always put in a new bulb regardless of what I am told about its age.
Gravel, I toss, decoration I drop in the beach water and clean up, but usually I toss most of that stuff too
I know this is more than what you asked for and you may already know this and do this....but just in case...I thought I would share as I have never bought a brand new aquarium......I get them for free or used...
Another good place to get free tanks especially in college towns, when school lets out for the summer go hit the dumpsters....wow....the stuff these kids throw out.........I have found 3 tanks this way and one was half full 55g with a fish ( common pleco) still alive in it.....