Definitely craigslist, also check free local classifieds online. Several craigslisters actually informed me they pick most tanks off of tree lawns on trash night.
I have had many a friends ask pet shops if they have any used in back to sell as smaller stores sell them off regularly if you ask. Finally, yard sales, I've got amazing finds- 150 gal paid $50
As long as you clean it well, these are good resources. Also you can strike deals if you see the seams loose inside or they say it does not hold water. I use a clear, waterproof , aquarium safe caulk and I've never had an issue even with previously leaky tanks.
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