I have found bags on ebay and using google:
14×26 Woven Polypropylene Sand Bags With Ties & UV Protection. lot of 500
Price:US $115.00 free shipping
I’m playing this a bit dangerously and plan to have a pile of sand delivered when I see smoke on the horizon. If I had a different layout around the house and property I would get the sand and cover it with a tarp. Getting a dump truck near the house anywhere but up the driveway is a problem for me.
I haven’t bought bags yet as I have a storage issue at the moment. I moved out of my 800 sq’ studio and have all my tools and stuff in my garage. I have a pile of 3 phase equipment and motors to sell taking up space.