On the "Backpacking Arkansas forum" someone posted a picture of a collapsible water bottle for $1 they found at the "Dollar Tree" store.

My wife told me we have one of those stores here so we went there today and bought a couple. They even come with a cheap carabiner.

When I got it home I tried a "Brita Bottle Filter" in it and it works perfectly. So, for about $4.50 you can get a lightweight 520 ml bladder type water bottle with a charcoal filter.

Before I take it on a trip I'll test it out for a bit to see how it holds up, but as far as I can tell it looks like it should do pretty good.