I am not exactly sure where you can find them. Try eBay, look under survival. Or military surplus. Perhaps even an boating supply shop such as West Marine. Since they are a government supply item, it will be in the US supply listings, and the manufacturer can be tracked from there. We used them in the Navy, and they worked well enough, but were not the most efficient device. Also, the water didn't taste very good, but dying of dehydration is a much worse alternative.
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If only closed minds came with closed mouths.