I take monthly trips to candy shops and party supply stores and stroll down the isles looking for PSK cases. It's amazing how many different types of candies come in hard packaging made of plastic or tin.
As someone stated before, they're probably not going to be waterproof unless they store some kind of liquid high fructose kid fuel. LOL
Today, at the party supply store I picked up a neat pack of
Invisible Cards because I wanted some waterproof cards for my BOB and when I saw these I loved the way they looked (I'm a sucker for cool and interesting graphic design). Unfortunately, the plastic case they came in would make an awesome PSK case as well and I may end up wanting to use it for that instead! LOL
I also check online candy stores, but most of the time they want you to buy in bulk.