I'm quite interested in that as well. Have just been on Amazon.com and you can get a set of 4 of them sent across from the US on 5-6 day delivery for about £63.00, so that like £16.00 each which is really good value!!
http://www.amazon.com/gp/product/B00FBGC...PDKIKX0DER
The only question I have for BP is how secure/safe do you think the two ends of the filter are? ie: are they likely to snap or break off easily when backed away in the depth of a backpack?
The only negative feedback on amazon seems to be about the bags, but thats not a big issue, as you can use it with most other bottles anyway....