I'm not sure there is a specific cover that fits with these strollers, I see most people with plastic ponchos. You could get a couple of cheap ones from your local dollar store and some clothespins to make it stay.
Not knocking your choice at all. Depending on your modes of transport and where you are staying, you may want to consider renting a stroller from one of the outside vendors, They will usually supply a rain cover with your stroller, or you can get it added. For many trips, it can be cheaper to rent from them than from Disney each day.