The FDA didn't ban it: it just cannot be marketed as a health supplement. Mostly because it was originally approved as a drug back in the 1960s. It would be like selling Tylenol as a vitamin. A dumb technicality that'll likely be reversed.
There are many brands listed on iHerb (all with "dietary supplement" prominently listed in the product description), though most of the items are marked "out of stock", so it looks like many people have been stocking up.
https://www.iherb.com/c/n-acetyl-cysteine-nac