If you’re having trouble finding NAC, try searching for n-acetyl cystine.
Also avoid Amazon. The FDA has been leaning on NAC distributors lately and Amazon responded by limiting sales. There’s only a few left selling at absurd prices.
I like Now brand from VitaCost.
NAC is also great at preventing progression of respiratory illnesses.
I use Thorne for NAC, Quercetin, and Zinc. I buy direct from the manufacturer. Their products seem to be very high quality and they provide a lot of information on how their products are manufactured and their quality control.
I, too, have used Now brand and thought they were good.
Amazon is notorious for letting merchants sell counterfeit products, too. I am more comfortable buying from the manufacturer.
Ever since college, I’ve gotten one to two colds per year with most of them progressing to chronic bronchitis that would last 10 to 12 weeks. Since starting NAC two years ago (plus zinc, Vit C, Vit D3 and lots of hand washing), I haven’t gotten a single cold and no bronchitis. Of course, having grown kids and being retired (out of an office) undoubtedly helps a lot, too.