I found this portable water container when I was at the Learning Expo in Chicago. I've been using it lately and I really like how well it works!
It's made from a heavy vinyl material, similar to that which those small, kid-sized swimming pools are made. However, it's extremely light, small, and flat when folded up. When opened, it holds about a quart (946 mL) of liquid. The container is about 4"x4"x4" when open (approx 10x10x10 cm). My favorite feature is the little pockets on the outside that hold brushes. When painting plein air, these brush holders really come in handy.
As I mentioned, I bought this at an art expo, and I haven't seen this item offered in any of my usual art catalogs/websites (Dick Blick, Daniel Smith, Cheap Joe's, etc.) so far. I bought this at a Princeton Brush booth, so perhaps they can be found through them. I'll keep a lookout and report back if I find a retail source.
Special thanks to my wonderful wife for being the hand model. :)