shok,
don't know where you can buy used uhaul trailers, but you can buy that type of trailer from a dealer or keep a look out for a used one on RCU or in your local paper. Also, if you don't mind driving there is a big trailer company out of georgia that sells pretty good trailers at a resonable price. Not sure of the name but if you look on ebay under cargo tralers their company sells alot there.

Patric.