N-acetylcysteine, or NAC, is an over-the-counter compound that can be purchased at your local health food store
Not for long. The U.S. just reclassified it as a drug. [1] I have a few bottles of it, but when they truly expire I will have to make my own.
[1] - https://proteanutrition.com/2021/06/08/fda-changes-in-usage-...