We purchased ours through babycenter.com, mainly because I live in the boonies and couldn't find anywhere near me that sold them.
Word of advice, my two year old twins FREAKED OUT when we put the cover on the double stroller. It was like a scene out of a low budget horror movie. Very, very, bad. In the thirty seconds I had it over them, they fogged the whole thing up and each tore a hole in it - and it was really heavy plastic material!
Maybe if we had used one when they were smaller, but it absolutely did not work for us! My DH and I each ended up carrying a girl under our ponchos, and just threw all our stuff in the stroller. They were much happier looking out of our face holes, which was better, I guess.