Oops I didn't see your dates, or answer your original question, sorry.
No they don't sell stroller rain covers at WDW. You can try the wallmart or target in orlando or use an adult poncho. IIRC the adult poncho is now clear plastic so if you place it properly the child can have ventilation on the sides and still see out the front. Just pack some clothespins, very handy for everything and you will be fine!
TJ
TJ