Where can i buy cheap, long hair extensions?

Im looking for either dark brown or reddish hair extensions for a party im going to dressed as ariel :) I have to have really really long ones as my hair is already red (dyed) and half-way down by back. Does anyone know any good websites that sell long cheap hair extensions as I will never wear them again and i hate paying the money when i know that they'll lie in a drawer until halloween! Thanks :) x