Classifieds--depending on what you want it for. Make sure they'll let you do a fill test before buying, and don't be afraid to ask what the tank was used for and when. I've seen people trying to get aquarium prices for lizard tanks--scary, since the lizard tanks have much thinner glass.
Otherwise--watch for sales. Lots of the big-box pet stores have $1/gallon sales, though usually only on the standard shape tanks.