Dri-rite at Lowes/Home Depot in the paint section. Get the refill. About $5-6 and will last you a very, very long time. It's a dessicant used to dry musty closets before painting.

It's similar to silica, can be recharged by baking it in an oven.. Just toss some in a 35mm film can with holes poked in the lid and sides.