I recommend Armageddon Gear bags, because I use them personally and I can vouch for their quality. There are lots of great options for support bags, but these will not let you down. From biggest to smallest:
Optimized Game Changer #
The OG. Great for stuff on the range when the priority is maximum stability. Big, but available with lighter fills depending on your needs.
Shmedium Game Changer #
The ideal competition size.
Pint Sized Game Changer #
A smaller option for those looking to take a Game Changer out into the field. Still big enough to sit atop a tripod or transform irregular natural surfaces into stable shooting platforms.
Squishy Rear Bag #
A simple and straightforward rear support bag. Lightweight, flexible, gets the job done.