We tend to reuse bottles rather than source virgin ones, since there are so many in the waste/recycling stream. For kits, I would have people find their own (same as the camera casing for the balloon maps).
Regardless, they should be square rather than round to avoid BBL spinning-- the square welch's juice container kind are our favourite. The rectangular shape is stronger than larger square shapes like distilled water containers.
Round jugs are hard to stabilize in the water and I don't recommend them at all. Moreover, it's hard to calculate how much water you've sampled in a round mouth compared to a square/rectangular mouth.
So the closet that snow found is this one: https://www.usplastic.com/catalog/item.aspx?itemid=121947&catid=611
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