to buy you need to know what you have. theres 2 different types of laser modules for phat ps3s - one has a single lense and the other has 2 lenses and they are not compatible. if its a slim i dont know the laser configuration for them. whether its a phat ps3 or a slim you need to open your ps3 and remove the bluray drive and open it to see what kind of laser is inside it then simply buy one of those.
youtube is your friend if you need help dismantling your ps3 and blu ray drive. one small tip - dont break the plastic clips.