Dollar Tree sells a two-pack up near their registers. Keep in mind these are the lightest, thinnest, wimpiest of all ponchos, but if you think of them as disposable rather than reusable, they might fit the bill.
I bought a handful for our next trip to WDW and plan to keep one in each fanny pack and the rest in the room. If we need or want them, we can just throw them away after we use them rather than having to worry about packing them back up (they're wet at that point and have somehow expanded to 20 times the size they were when the factory folded them up). Next day, we just reload as necessary. Size-wise, folded they are a little bit thinner than a pack of tissues.
Personally, I have never used a poncho in the "real world" so I'm not big on buying one I can reuse later. But that's just me.
My two cents. . . Happy planning!