either petsmart or petco has a bunch for a $1.00 each right now
Those are the cheapest, but almost every fish from the Petco by me has had ick, not that privately owned LFS don't have problems, but usually their fish are less sick.
Fins and some other places have very healthy fish so especially if you have any other expensive fish in the tank already, I wouldn't risk adding Petco/Petsmart fish unless you quarantine them first for a week or several weeks.