I bought my Harry Potter ones from Sam Goody's in my local mall. They were about $5 (of course they aren't the park ones, but I got Gryffindor with a rubber crest and Hogwarts themed with a rubber Hogwarts crest.)
Also, I've seen Spongebob and Marvel ones for $1 at Party City (formerly Factory Card Outlet around here.) Those ones didn't come with a pouch, but I order our pouches cheap off
Amazon, so we can have the ones with the ziplock style closure. (Water rides will soak you

)