For long term use, I'd buy it on e-bay. (just make sure it's a good grade), buy some bags and make your own. I buy it by the 5 gallon bucket for about $30-35.
Is that the stuff that absorbs ammonia? I never under stood that crap if your tank is properly cycled and you have the right amount of bio media then you will starve your bio media possibly. If you are cycled yet over stocked and relying on ammo zorb that is stupid