i would try a mix of clean top soil (with no additives or clean dirt from your backyard) and peat moss. both can be found at gardening stores for cheap. coco coir from pet stores is expensive and is the same thing as peat moss.
I would use a mixture of the coco coir and the coco husks. They both hold moisture really well. Just be sure to keep it moist. That way it won't get dry. If it gets too dry it will get dusty and will more than likely impact them. This picture is of my redfoot, Roxx. It has the coco coir and husk mixed in. It is very soft and She loves to bury in it.