I'd check e-bay but why get one? A milbank bag, as I recall was a gravity fed water bag to filter out big stuff but no small stuff and certainly not crypto or giardia. You would then need to add iodine or boil the water.
IMHO, unless you have an affinity for it you have better options. Heck a bandanna and a non-potable nalogene bottle would do the same.
But you can probably find a Milbank cheap and it packs small.
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