Do you mean for toothed belts like from Gates, or for the polycord (round urethane belts) teams often use in their collectors?
For the polycord, you can just cut and remake the loop shorter (heat if solid, barbs if hollow).
For Gates belts, you can probably just make your own tensioners. Heres a picture of 1625s tensioner in a gearbox
http://www.chiefdelphi.com/forums/sh...6&postcount=33
I don't know how well this idea worked, but another team just used the andymark chain tensioners.
http://www.chiefdelphi.com/forums/sh...elt+tensioning
Keep in mind I don't have a lot of personal experience with either of these two methods, I just saw pictures.