We forgot our Britas on our last trip, and discovered that they are pretty much NEVER available in Orlando-area stores. We tried 8 different places, and all were sold out. Elsewhere in Florida, yes, but nowhere around Orlando.
I've never been able to find any at a Target anywhere, but most WalMarts will carry them, at least in the summer. Many supermarkets have them in the bottled water aisle, near the really large bottles, usually, and usually on the very top shelf.
Also, you might try the big chain drugstores, like Walgreens or CVS. The box is dark blue with a life-sized photo of the white bottle on the front.
Culligan also makes wide-mouth filter bottles, and I've noticed that stores always seem to carry one of the other, but not both brands. Rubbermaid was making them for awhile, but I think they got out of the market; I didn't like them anyway, they did not have a wide-mouth top you could use ice with.