I stock up on the 2 for $1 at the Dollar Store. Like others have said they are really just a one day poncho bc they are pretty cheap but when they rip I don't feel bad tossing them in the trash. We carry them in a ziploc bag in our backpack. If it is a quick rainshower, we take them off and put them back in the bag in case we need them later.
We were in WDW on a very rainy day last year (rained from 8am to 9 pm nonstop) and our Dollar Store ponchos lasted the day and kept us all dry!