I'd imagine the back of any connection Tamagotchi would do, seeing as the molds are quite similar and I have seen people swap them. When I say "connection" I mean versions 1-5.5 and also the Japanese connection Tamagotchis which have the same shells (Project TamaShell is a good refference website).
In light of your other thread, a casual collector or anyone who has a few extra Tamagotchis might not be bothered by a missing back cover, as they could swap back covers or make one out of cardboard. The back cover isn't terribly crucial for a connection Tamagotchi as the battery stays in place due to the specially molded slot, thus not interfering with game play. Additionally, it's rarely seen in normal use, thus making it easy to forget. It isn't really feasible to buy a a broken Tamagotchi for parts as one might sell for $20 but they are not really discounted from the normal price range of 30$-60$. But with a back cover, you could probably sell your Music Star for likely $40 as the shell is in good condition. Bare in mind, the prices I have listed are in Canadian dollars since that is my currency and I use the Canadian Ebay, so you may wish to convert them if you use U.S. dollars.