Old cotton clothing and rags work very well at stopping arrows.
Save those old tshirts, jeans, and socks that are too worn out to be used as second hand clothing. Stuff them into a burlap bag or cardboard box and use them as archery targets.
That's what I used to do. I would stuff all of our old, completely worn-out clothing in a large cardboard box. When it was full, I'd tape it shut and use it for archery practice. When the box starts falling apart, just get another box and transfer the old clothing. The now-shredded clothing is still good enough to keep stopping arrows, but it will have settled a bit, so throw in some more old clothing and tape it shut again.
I've only shot cardboard boxes filled with old clothing. I'm sure a burlap bag or those plasticy potato bags would work very well, too.
I've only used cotton clothing. I don't know how other materials will hold up.
Very important: This works just for field tips. Don't ever use broadheads on a target filled with old clothing. In order to remove your arrow, you'll need to unscrew the broadhead - which will require you to open the box (or bag) and dig (and probably get a severe cut).