http://www.ballroomcostumes.com/
This site specializes in lightly-used dance costumes for students on a budget. Also, you may want to try checking the classifieds or a dance instructor. I bought my drop-dead gorgeous smooth gown (used) from an instructor for 500 USD; my rhythm dress was made by a local woman and had 2 previous owners: and still looks brand new (also 500 USD)!
Good luck finding your dress
