Well...I don't think it really matters,but traditionally..long is for prom (more formal) and shorter and cuter for homecoming. I guess w/ modern style it doesn't matter at all.
in my area we have a thing called "project fairy godmother" in which all of the high schools collect from students dresses that they donate and then later they are all put together and sold at a price of i believe $10 to those who cannot afford expen...
hmm try www.promgirl.net or hope you find something nice! x