Where to buy derailluer for shimano havoc

[franksevers]

I bought this bike for my son at Walmart. He says no, but he certainly dropped it. the rear derailluer is destroyed. its made with plastic parts. I cannot find online a replacement. And i don't know enough to make a choice for a different one. can someone help me? I don't want to spend a lot on any thing too fancy. I just want my son to be able to ride.
PS: It is a Shimano Havoc Hyper 26" frame. The part reads rd-tz31

[nfmisso]

You want one of these:
http://www.amazon.com/Shimano-Tourney-TX35-Derailleur-Speed/dp/B003ZMH69S/

The TX31 was discontinued 3 or 4 years ago, and replaced with theTX35.