Derailleur hanger on frame?
If you're on a super tight budget you can cut up an old derailleur to act as a tensioner. Alternatively perhaps something like a Surly singleator.
If you don't like either of these alternatives as they muck up the clean SS chainline then these chainstay rollers are a bit more subtle:
http://ecx.images-amazon.com/images/...500_AA300_.jpg