The tensioner is expensive, even from Tischer. You can get both for half the list price from places like RMeuropean (although I'm noticing it's out of stock -- you may need to wait a bit), just google the part number. Don't forget to order the bolt, it's one time use only. Best to replace both pulleys, and the belt if it hasn't been done.
http://www.rmeuropean.com/Part-Numbe..._85E85BAC.aspx