Buy.com has PlayStation 3 Wireless Controllers for $29.99 if you use Google Checkout for the first time.
If you have never used Google Checkout with Buy.com you will get $10 off your first time when you pay through Google Checkout. They have PlayStation 3 Wireless Controllers on sale for $39.99. I am not sure how long the sale will last but that is by far the cheapest I have seen them.
PlayStation 3 Wireless Controller
FEATURES*
Control features two thumbsticks, D-pad and six buttons
Connects via bluetooth (wireless) or USB (wired)
Lithium Ion rechargeable battery built-in
Battery running time - About 30 hours
Play & Charge technology - Recharge battery pack while connecting via USB
