Easy - find a cotton handkerchief or bandana that someone littered

technically natural, and functional as a sling.
All jokes aside though, I'm interested in having a go at it. My videoing skills stink though

Nettle fiber can still twist up nice and tight while still wet, and has a lot of its strength. It's not as strong as when dry, but quite adequate for throwing a few stones. But where I'm at, if the fiber is hung on a fence or on a branch in full sun, it'll dry really quick
This one time I made a Balearic style sling from the terrible awful foxtail grass that grows so abundantly where I'm at. Was about this time of year, made it in one day, and it lasted for a few stone throws. That was back when I started braiding slings though, so the braid was really loose, and it was a three-strand braid.