I've tried everything for mini moo card storage: the various holders you buy from Moo, gum tins, mint tins. Everything.
Then, in The Container Store in Houston, I found these:I just thought they were cute; I had no idea whether or not the cards would fit. But! They fit perfectly! Yay! Plus they hold about 25 cards, WAY more than any of the others. And they have a neck strap, so if you're at a convention or art retreat or somewhere where you want to have cards handy but have your hands free, this is PERFECT. And only $5. Plus it seems pretty sturdy, too. I got one of these and one of the larger, regular-card-sized ones for my Actual Business Cards. Cool.