I have the opposite problem to you, i am tall and its hard to find them long enough. I know though that Principles and Dorothy Perkins do a petite range.
I have that trouble too. Primark skinny jeans for instance are a good 6 inches too long. I have managed to get cheap jeans from tesco and asda. Principles do a petite but they are dear. Check out fashion tips on GMTV or this morning for help.