Hi clearwater,
If you mean plastic locking flip-top containers try these Lock & Lock ones by Starfrit,
http://www.starfrit.com/Products/kitchen/Container/Lunch%20kit/94628.aspx?sc_lang=enI have a 350ml (11oz) rectangular one in front of me now, it would make a good small PSK container which is why I bought it. I got it at Walmart, they are sold in sets or as single containers for a couple of dollars. The lid snaps tightly in place and has a seal, the label says it is "absolutely airtight, waterproof and freezer, dishwasher, microwave safe.
I think one of these containers was what Ray Mears carried his hot chocolate powder in when he was in Africa camping with the Hadza (there was an ETS Forum post about the container after the show aired).
The only thing I do not like about sealed plastic containers as a PSK's is that you cannot place them directly over a fire; to get around this I lined the inside of the plastic container with a fitted heavy-duty aluminum foil baking pan (like a mini-loaf pan). On one version I even went so far as to reinforce the outside of the fitted pan with metallic muffler tape to strengthen it (have not tested it yet over a fire). This way you get the benefits of the sealed container and have something to boil water in (although a little fragile, but better than aluminum foil.)
Mike