i think you're going to have a hard time finding natural material but check with DEWFOAM which is behind hotel Essencia on Ma Christina st.
https://goo.gl/maps/QQwBXZvVCQMJoaQx7

used plastic bulk item sacks used to be available from venders that sell various used bulk containers. i'm out of touch but there used to venders on the inside of the curve between rob mall and the park.
https://goo.gl/maps/AopXBeiaUVu39o2j7

and the outside of the same curve near the intersection that goes to the water front.
https://goo.gl/maps/KvEKD4aFndrE7H2b7

 

ricebags?