I'd start calling up local parts stores and see if you can order it. Or hit up a few local dealers and see if they have any left. Other than that just keep searching 12596250 on the net and see if one pops up. Often I'll see them on Ebay.

Worst case hit up a junk yard or 2. IIRC these tensioners were used on any 5.3L FWD car (Monte Carlo, Buick LaCrosse, Grand Prix). In fact you may give car-part.com a try and see if you can find a used one.