Good for you, my plus sized friend did a sprint with me last year, she was not laughed at but cheered by people more than anyone else! It is a real accomplishment to run with extra weight and shows STRENGTH.
She recommends "googling for Danskin plus size activewear- I bought my tri-suit from Sports Authority online last year."

Four other sources!

www.teamestrogen.com
www.athleta.com
www.titlenine.com
www.paragonsports.com