The bike came with a claw derailleur right? Just pick up something cheap from your local bike coop. $5 and you're good.
You're talking about this bike, right?
http://www.bikeboompeugeot.com/Broch...hure%20P18.jpg
Pretty much any rear derailleur with a claw will work. They're available and cheap.
Now if you're talking about a bike with a forged simplex rear drop out, then finding a RD just became a lot harder.