I would run by the dollar store and get them there. That way you can guarantee you aren't paying an arm and a leg. Everyone else will have their ponchos priced tourist-trap high, more than likely. And while the dollar store ponchos may be a little thin, if you aren't too rough with them, you'll be fine.
Or you can do what we do...just go with it. Walk through the rain (hit every puddle along the way) and wave to everyone who thinks you are insane! We used the poncho for the stroller, and we all got soaked! (We did change DD's clothes before going into air conditioning, though).
One suggestion...put your valuables (cell phone, camera, wallet, paper tickets) into a ziploc bag any time it starts raining or when you go on the water rides. MAJOR life saver!
Have fun, and don't let the weather spoil it! It's part of the fun of WDW!