endless bike co. makes one-25$
they make sprokets too...
im not sure although if you look around i'm sure you will find others as well
if you have vertical dropouts you can either try to run a magic gear(click here for info on how to calculate)
or you can use a chain tensioner if you do I have heard that the rennen rollenlager is one of the best out there. I have used the surly chain tensioner and it works just fine though