Isn't it just a generic one, the power brick is built-in for the slim PS3? If you want a local store, gamestop might work.
I think the cable you're looking for is a Nyko power cord for $4.99 on Gamestop's website, so you can check in-store. I could be wrong if it doesn't work with a PS3, there is another Nyko one for $9.99 one that says for PS3.
I'm sure there are cheaper places online and maybe in-store like Radioshack or Walmart for this particular cable.