I concur that I should be able to get this at Home Depot or Lowes. I have checked all stores in my state and no one carries it at the moment. The closest store that does is a few hours away.
The only bags I see are these. Unfortunately, I need 10-12cu ft to fill the whole enclosure. If this is the only thing I can find, then I’ll get it. I was under the impression that the substrate should be changed monthly-ish.