Home Depot or Lowes FTW!
10 for 20$ but once you factor in shipping its probly cheaper or about the same price to get 5 gal bucket from the Homeless Dumpo or lowes.
http://www.htgsupply.com/Product-TEN---5-Gallon-Plastic-Nursery-Pots.asp
Do the aesthetic qualities of the pots matter? because if not just use buckets with a ton of holes drilled in em, about $2.50 a pop at casa de pot
Gotta love that classic home depot orange...