Did you need the radiator? Or was it the heads? If it wasn't the radiator the mechanic should reimburse your money and put your old radiator back on. His mistake.
If it was the case of having a hole in the radiator and because of running out of water you damaged your heads,looks like you have no choice. Unless
you go to http://Schmidty.Aidpage.com , http://soulight.aidpage.com , and http://starshine.aidpage.com to see if there are any free vehicles in your area so that you can replace it.
You can also put free cars in your search engine and see what it comes up with. It would show some in your area, or it should.
Hope this helps. Good luck and God Bless