The Culligan bottles are also discontinued, I have some of those. (I don't like them as much as the Brita; the neck isn't big enough to take ice cubes easily.)
If you can't find a store that still has the Britas or Culligans, there are a fair number of them available through merchants on Ebay. It has been nearly a yr. since they were discontinued, so they will be REALLY scarce pretty soon.
There is one other lower-priced personal filter bottle that I know of, and I don't know whether or not that one is discontinued, too. It's from Innova. It was always harder to find, but you can buy it on the web. (BTW, Innova sells promotional versions of this bottle, maybe we should get it added to the DIS store?)
I think that what happened with these bottles is that the companies probably ended up with lawsuits from people who got Giardia, etc. from using them with untreated water, such as from streams. These bottles were never meant to filter out harmful bacteria, just minerals that affect flavor. Bottles to handle that more serious filtration (so-called biologic purifiers) *do* exist, but they are MUCH more expensive than this type, about $50 each, with replacement filters for around $30. I suspect that with the portables, the risk of people confusing the two types was too much of a liability.