The cheapest way is to find your closest aquarium club and buy from their members (either a private sale or an auction).
I'm not sure how far you are willing to drive, but if you are willing to drive a bit, you can find some awesome fish from other hobbyists. Although the price of the gas can negate the savings.
I was just at Atlantis about a week ago to pick up some Apistogramma. The amount of juvenile African cichlids they have is overwhelming. Tons of everything and the fish were all super healthy and good looking. I'm not into African's myself, but I was impressed with all they had and how well cared for the fish were.