I want to use one of these with my Adams piston build, but cannot find this anywhere (even Armalite is out of stock). Can anyone link or guide me to where these are available? Thank you.

Call your local gunsmiths and gun shops. I was working on an upper and needed shims, ran across the same problem, out of stock or outrageous shipping charges. I found a handful locally, and for free no less.

EDIT:
My shims were for the muzzle device. I did make my own shim for a gas block by taking an old handguard cap, and trimming it down with tin snips and a dremel tool. I even left the upper hole on my shim for the gas tube, helps keep it in place. Old handguard caps should be easy to locate as well.