Go to your local shooting supply store and ask for #7 or #6 chilled shot. They typically come in 25 lb bags. It is for shot shell reloading. If you can’t find it locally, try midwayusa.com - it is a mail order reloading supply house. look for shotshell reloading components.
I used 7 shot for mine and made my bags out of window screen material. 10 years later and they are still in service with minor repairs. The screen material makes it easier to wash the mud off and drain easier than if you use noeprene or lycra.
Steve