You have several options to buy the PS3.
You can visit sites such as
www.elmazad.com and look for used ones for relatively cheap prices. But you're going to have to buy a used one with all the risk that entails. Although the PS3's build is of high quality so a used one may not be that bad.
I recommend that you go buy a new one for several reasons. First, its new so you'll be having the best build quality. Second, you'll be recieving the latest upgrades in the hardware configuration.
There are 2 places I know of: Nino and Virgin megastores. There are definitely other places to look for but these are the ones I usually go for my gaming needs.
Good luck....