In my experience, the 3rd worst on-line shopping experience in the world is Wal Mart online (the first two are auto supply houses, and HomeDepot/Lowes). I can't find it either, but they are usually in the actual store, in or near the camping section as you thought.
You can see them at REI here
http://www.rei.com/product/618168
but they are cheaper at WalMart. They sold a ton of 'em in advance of the Y2K scare - remember that? Somewhow when the calendar rolled over, water would no longer flow out of your faucet. Anyway, I bought 3 of them for $2 each from a guy who didn't want to admit why he had them.
Anyway, even at REI they are a pretty good deal. The square shape makes them easy to carry, and the opening is big enough to accept the REI bilge pump, that has been discussed in other threads. And the fact that they are rigid also has some advantages - I also have a couple collapsible water jugs, which also have their advantages.
Bill