Try craigslist sometimes to. I know there can be some shady stuff on there tho, but I found a 120gig slim for $50. Ad said the dvd wouldn't play anything. Took it apart and ended up being two blurays stacked on top of each other on the inside along with a puzzle piece wedged on the laser track. Worked good as new after removing. Sometimes you can find deals like that.
You can also try repairing your fat ps3, look around online for tutorials.