I think REI has the Platypus canteens. There's probably an REI near you. I had the 1L version a while back and IIRC it sprung a leak. They're certainly more durable than ziploc bags, but much bulkier too. I'd say you can't really put one in a typical pocket kit. I guess they could be handy in a larger kit but at that point I'd go for the 2L version or an even larger one. Nalgene also makes some canteens like that, up to 3 liters. See rei.com for a wide selection.