If you don't need it for display, go to your local petsmart and ask if they have any of their discard acrylic display tanks for sale. I got six of them for $5.00 each. Apparently, the employees tend to scratch them while cleaning and they replace them fairly regularly.
The small rectangular ones (24x12x12) are 15 gallons and they sell 'em for $5.00 with fittings (1.25" Drain, .5" return with bulkheads) they also have a larger size, but I don't have the specs on that one...
HTH,
RandalB