Reddit mentions of Officemate Pencil Style Moistener with Wedge Sponge (97802)

You could use something like this (or this), but they don't have any kind of cap, so you'd probably still want to put them in a Ziploc bag or something, to feel safe about tossing them in a bag. Or you could just keep a bit of damp sponge or rag in a plastic bag or in one of those little plastic containers like people use for condiments or snacks, as long as you remembered to rinse/re-wet it often enough.