UK or US......
I keep an eye on Fleabay, Craigslist, Gumtree and Freecycle (in the UK)......I have never bought new...and always had excellent bargains.....bootsales can also turn up the odd tank (£20 for the last Hagen 3ft I bought with Eheim Pro filter and all pipework...sold the tank on for £40 to a mate, and upgraded my filtration).
Again in the UK....Aquarist-Classifieds are pretty good.
...oh...and forums are good....any forum...just keep an eye on the "general classifieds" section of pretty much any forum for bits and bobs...
.....and word of mouth..........let it be known that you are "in the market" for fish tanks.
Last year, I also put an ad in the local paper, saying that I was looking to buy cheap fish tanks.....got a few and sold a couple on Ebay...didn't make a mint, but a bit of cash coming in pays for fish food and consumables........it is also a bit of fun too.