If you're talking about the "T" bar, I got them in a large bionicle set a while a go, but if you are talking about the two hinge pieces that are hooked on too the "T" bar, I've gotten them in a bunch of different sets. If you are trying too find pieces, try bricklink.

really cool! where do you get those pieces you used as hinges, on the feet of the walker?

Great little MOC!