Hope you aren't spending gobs of money on little pieces from ISC.
This is what it is made of (click link below), and you will have a life supply for $10. Just cut the shapes you need.
Super Shammy (available at auto parts stores and discount department stores)
PS. To answer the question. Washing is fine, until the fungus grows. You won't kill all of it and you don't want it getting in your lungs or breather. Washing with bleach will kill it all, but will also wreck the material. Just replace it. With bulk material, you will be able to replace these things as often as you like. Also, I would not dive the unit without one in place. They are super absorbant and are a line of defense against moisture on the cells and minor floods. Paper towels when saturated can come apart and make a mess inside the unit, as well as create blockage. Cheap paper towels leave lint as well.