If you're getting something like a 3 fingered case, and you want to keep humidity in it, I'd get a telescoping one that sort of locks down when you slide the lid over it. That will keep it air tight. The sponges are really the only stock option in those kinds of cases, which are crappy but you could always use beads out of THE PUCK tied up in a bit of panyhose or you could put a water pillow in there. Remember, if you take them out of the humidor and put them in an air tight case, they will stay fresh for a week or two if you don't open the case.

Otterboxes are also a good choice for transporting more cigars.