I always go oem on that kind of stuff.
I think gates brand are good. Any parts store might carry them. I think Napa carries Gates.
http://www.gates.com/brochure.cfm?brochure=4956&location_id=544
I just replaced all the pulleys on my e99 and got gates from orielly auto parts
Motorcraft single pulley tensioner (BT-44) is $76 at http://RockAuto.com
You could just replace the bearing, cost about $5.
You could just replace the bearing, cost about $5.