Jeeze, all these rich guys want to BUY stuff. Us poorboys have other ways and means.
Go to most any construction site or bulding supply yard and ask if there are any dryrock/wallboard scraps. There will most always be. Drywall has to be trimmed. Sheets break often in storage/loading/unloading. If you ask nicely, folks should be happy to give them to you just to get rid of them.
Gypsum wallboard is very hydroscopic. Absorbs moisture.
Cut the scraps into pieces about the size of a playing card. Peel the paper off one side and bake in the oven at about 150 degrees for an hour or two to dry it out. You can then put it in an ammo can or other sealable container and the drywall will absorb all the mositure within. You can re-dry it almost an unlimited number of times.
Spend your money on ammo, pilgrim!