Chains, any seven speed chain, I use chains other than shimano, Sedis, KMC and the KMC quick links. Quick links make chain breaking and making without tools easy, once the correct length is obtained that is.
But as others have said, maybe time to change chain rings and cassette as well otherwise put a new chain on old worn out stuff, your drive train won't be up to much.
Biopace rings can still be got, seen some new ones advertised on the bay recently. Cassettes again easy and one does not necessarily have to go with Shimano, there is some tougher stuff out there.