Do a google shopping search for "CORNY KEGS" or "BALL LOCK KEGS."
There are about 50 places that sell them online, new and used.
You should pay about $40-50 used or $100-120 new for 5 gallon ball lock corny kegs. Most brewers buy them used. You'll need disconnects, tubing, a CO2 tank, and a CO2 regulator if you don't have it already. Lots of these guys sell the whole shibang in a set. Keg Cowboy in the vendor section of this site has great prices if you need to buy all of the components in a set, though they can be beat price wise if you just need individual kegs.
Check out Ebay as well. Tons of vendors there as well, mostly the same ones, but sometimes a bit cheaper on Ebay for some reason.
Check out your local home brew shop as well. Sometimes they have them cheaper than online once you factor in saving shipping costs.
Good luck!