@sueinmn Here is a list I had Gemini (I subscribe) gave just gave me. I use Pure because I could find it easily.
Pure Encapsulations (NAC 600 mg / 900 mg) A practitioner-grade brand recognized for hypoallergenic formulas, free from artificial additives, common allergens, and unnecessary binders.All raw ingredients and finished lots undergo third-party testing via accredited independent labs.
Thorne (NAC 500 mg)Holds NSF Certified for Sport and cGMP-compliant manufacturing standards. Heavily vetted by healthcare providers and athletic organizations for identity, potency, and absence of heavy metals or contaminants.
NOW Foods (NAC 600 mg / 1,000 mg) Comprehensive in-house testing combined with third-party certification through Informed-Choice and UL/NPA cGMP. Widely accessible, consistent, and cost-effective.Life Extension (NAC 600 mg) Provides Certificates of Analysis (COA) for product batches upon request.Formulated using pharmaceutical-grade standards in GMP-certified facilities.Jarrow Formulas (NAC Sustain 600 mg) Features a bilayer release format (one-third immediate release, two-thirds sustained release) designed to maintain circulating levels longer. Third-party tested for purity and microbial/heavy-metal contamination.

(Note regarding USP: While finished dietary NAC supplements rarely carry the consumer-facing "USP Verified" seal, reputable brands source USP-grade raw material, which meets United States Pharmacopeia standards for chemical purity and identity.)