It's not a part, it's a tool. Torsion bars are not length adjustable, thus necessitating a stretcher. My local Harbor Freight stopped carrying them about 6 months ago, but the dealership I work at has been selling them over the front counter (parts department) for as long as I can remember. GM sometimes supplies bars that are a little shorter than spec, so we give them a bit of stretch before they get delivered to the customer.

I think our parts department sells the stretcher for about $70 plus tax (California <img src="/forums/images/graemlins/baby.gif" alt="" />)