As long as a cheese is called Parmigiano Reggiano (and not just Parmesan, Parmigiana, etc.), it's required to be made of just milk, salt, and rennet.
Where to find it locally
Costco (under their store-brand, Kirkland).
Trader Joe's.
Specialty cheese shops, Whole Foods' cheese area, etc. will often have it, potentially packaged under a brand like BelGioioso. [How consistent are other stores with mine?]
If you live in a major city, consider using Chowhound Discussions to collaborate with other locals to find the cheapest or highest-quality sources.
Amazon carries several sources, including igourmet listings from other sellers that are currently cheaper than ordering direct, with their "Top Grade" 1lb, 3lb, and 10lb options.