They sell it at most pet stores, even garden centers.

I know they have plenty of it at Petco, if you have one near you, i would take a little trip over there.

Also, when its time for shedding, i just cover a little more than half of the top and mist once a day, it stays at 80 percent, when the snakes are not shedding it still just stays at 60, i really dont have a humidity probem but when i need to raise it, that works,also filling up the water bowl up the the brim raises it a bit.