In the past year or two, the only "good" deal I've found on DualShock 3s would be to buy on Amazon. Wait a few days and wait until the price gets to its lowest point (or use a price tracker), because the prices fluctuate wildly, going up or down a few bucks on a daily basis. The lowest I can remember seeing there in recent memory would be about ~$39.
Alternatively, you could try buying one of the combo packs that include the controller + a PS3 game. If you play your cards right and are able to find a buyer to whom you can sell the PS3 game near full price, your net cost could be as low as $30-35 for the controller. Example: looking at Amazon, the God of War Origins collection costs $35, while the God of War Origins collection + DualShock 3 combo costs $71.