Is there any refrigerant currently in the system? If so, take it to a shop and have them evacuate the system, then you can replace your compressor. Make sure to replace the O-ring seals on the lines in and out of the compressor, or you'll have leaks again.
Then take it back to a shop to have them recharge it.
As far as getting a used compressor, you're better off coughing up a little more $$$ and getting a new or "refurbished" one.