Does it still work?!?
Replacing the frame may be pretty tough. From what I gathered when I fixed my home button on my iPhone 4, it's a MAJOR part of the phone. It's not just a part that goes around the outside; the entire phone is assembled around the heavy frame.
If you were really lucky, you might be able to find a water damaged phone family cheap that has a good condition frame, but I'd guess that it might be hard, considering that you could take a pretty messed up phone and buy a replacement at an Apple store for $200 or so.
If your phone is smashed but still in one piece, I'd see about going that route. $200 for an out of warranty replacement would probably be cheaper than buying parts to fix it.