"What a great idea! How much are they? Where can we find them? Do they come with a strap so I don't have to "carry" them?"
Brita 24 oz. filter sport bottles run about $6-8 dollars each, depending on where you buy them and if they are on sale. Do buy them at home, though; we forgot ours on one trip and tried 6 stores in Orlando to replace them, but they were all sold out. Most general merch. stores like Wal-Mart, K-Mart, Target, etc, or large drugstores like Walgreens or CVS will carry them. Sometimes grocery stores have them, too. The filter is replaceable, and each filter is good for about 15 gallons of use. The bottles don't come with a strap, but they have an indentation that goes all the way around about 1/3 from the top, an inch or two under the wide-mouth cap. You can take any piece of ribbon or webbing that's about 1/2 inch wide, and slip-knot it around the bottle at that point. Since it is a squirt-squeeze bottle, you don't really need to have one for every person; we share.
Culligan also sells a similar bottle for a similar price, but they are not as widely available. I think that one is slightly larger, around 30 oz. The Brita fits in most auto cupholders, I don't know about Culligan.
Edit: After writing all that, it occurs to me that you might have been asking about the neoprene holster instead of the bottle.
I got my belt holster at Target some years ago, it looks a lot like this:
http://www.uspolyinc.com/neo7.html You can usually get them at places that cater to campers and boating enthusiasts.