There are many retail stores where you can buy PS3 controllers for a decent price, but where can you buy them online for even cheaper you ask? Well, for starters, I would recommend looking at Amazon.com, since they not only have really good prices, they also have really good customer service and support, along with an awesome return policy, should you have to return the controller in the future. You will also find that Amazon has tons of PS3 controllers for sale, and they even have some cool bundle deals too, such as their god of war 3 bundle. If you buy that, you get god of war III plus a PS3 controller too.
Ebay is another option; they have really low prices too, but you may have to deal with online auctions, unless you find a buy it now auction, which will let you buy the controller instantly. They too have a good deal of PS3 controllers on sale too, so they may be worth checking out.