Calcium chloride as mentioned by mlts22 is what dri z air and damp rid use as their main ingredient.
I used to use CC as a drying agent in our bird baths and patio stones. When we got our first TT someone suggested dri z air for the very damp Washington coast weather. When I read the ingredients I was surprised how much they charged for 2 ounces of calcium chloride compared to the 50 pound bags I bought for around $8.
I would take a small burlap sack and put a few pounds of cc into it then set the bag on a piece of heavy wire mesh over a 5 gallon bucket and it worked great. One time it worked too well and the bucket I was using was about 2 gallons and had started to overflow. You have to check it every month or so. You also have to add some to the bag once in a while.
I was able to find 50 bags at my local cement plant, now I am able to find smaller quantities at my local tractor dealer as they put it into tractor tires. They all seem to have a partial bag laying around somewhere.