We just came back from the vet for a checkup and her comment was "yorkie's are so smart and have knack for training YOU". Boy isn't that the truth? The only advice I can give is to offer a wide variety of toys. My love the toys with the squeakers. Ours have stuffed, balls, harder plastic etc. OR.....you could get a little brother or sister!!!
Our local Mud Bay Granary carries these bells and I've seen them at independent pet shops too. If you can't find them locally try Petedge.com, I think they carry them.