I am not sure. If you have a rental car you could drive off site and buy one. On our last trip we did not have one and we used a Poncho, which worked ok. You have to fiqure out how to put it over the stroller properly so it does not get wet.
I have read many threads where people talk about putting an adult poncho over there stroller. I first year I went I took my rain cover and it was a pain to carry around. After that I said I would just buy a poncho. Turns out my dd's favorite part of Disney is getting rained on.