You can buy bulk silica gel at many websites or stores.

Hollinger Metal Edge is a company that sells museum and archival supplies:

Craft stores like A.C. Moore sell silica gel for drying flowers for use in craft projects:

I use the craft store variety. It's cheaper by a factor of four.

I get small plastic drinking cups and fill them about half way. Then I cover with a paper coffee filter and secure with a rubber band. Everything is sealed inside metal ammo boxes with tight fitting gasket lids.

I figure one or two cups full of silica inside an ammo box, depending on size, will protect me from almost anything short of a flood or fire. Right?