I bought this sling for my first Appleseed. It performed well for me and I didn't experience any slippage. Fittings are tight and seemed to be of good quality.
https://www.ammogarand.com/amm1gablcowe.html
I believe that is who the Appleseed store uses to supply cotton slings. They are a buck cheaper in the AS store, but don't know how it works out after shipping.
Check out military surplus stores in your area.
aj
The CMP store usually carries them as well at a good price.
https://estore.thecmp.org/Catalog/Item/854WEB
I usually place a bulk order in the spring to have them to sell to the students.