An Economical Way to Make Them
Take an old pair of jeans and cut the legs off. Normally one leg will make at least two or three sandbags. I normally will cut the legs into different sizes. Most men's jeans have legs that are approximately 24-27". I'll cut one leg into two or three pieces. Different size bags are better as you can position them as needed depending on the environment.
Sew one end of the leg completely shut using several lines of thread. A sewing machine works great as you can run the end back and forth several times. The stitching needs to be strong and have several lines of thread on each end. Next sew the other end several times almost to the end leaving enough room to get a small funnel end into the hole. When your through with the sewing, one end should be completely closed off and the other end with a small opening approximately an inch long.
Standard blue jeans work best as they are thick and very durable. Chamois cloth doesn't work as well. You can pick up a pair of jeans at a yard sale, thrift store, etc.. as any size will work.
Buy a bag of sand at a hardware store and fill the bag using a funnel. When it's full, either sew the hole shut or use several safety pins to close it off. Do not fill the bag completely full, only about 75%. This will allow you to position the bag and allow the weapon to work down into the sand. When I place the bag on the bench, I'll hold the bag up by one end and then curl the empty portion around the bottom. As you work the weapon into the bag it will allow the bag to expand.
I've used sand from an open pit and it doesn't work as well as the "Kids Play Sand" they sell. The open pit sand tends to be moist even in dry weather. You need the sand to be completely free of any moisture.
You can make many bags for less than a dollar each. I have a big plastic carrying tub with a top on it that I transport them to the shooting range. Make sure they don't get wet!
You could even make some and place in shooting houses that could require a long shot. We have some locations that you can shoot as far as you dare ... 800 yards or more. Be sure that they remain dry in the shooting house.
Make about 6 or 7 bags ... you'll really enjoy the shooting. I like to place 3 or 4 bags under the gun forearm and a couple of smaller ones under the trigger hand. I also take one bag and place it flat for my right and left elbow.
People say, "I don't have sandbags when I hunt" - that's true but shooting off of sandbags will allow you to really get a grip on how your weapon is performing - it allows you to fine tune your range and skills especially the trigger pull.