of course I can't suggest you any place in US where you can find Parmesan, but maybe can help you to choose something true and good.
The generic name of the cheese like Parmigiano is "Formaggio Grana", which indicates not only the Parmigiano Reggiano (which is the best and oldest one, born in the Middle Age in the city of Parma) but many similar types of cheese, made in other places in Italy and abroad. Many years ago, there was even a Grana cheese made in Argentina and named "Regianito"!
Apart from Parmigiano Reggiano, the only other which has a reliable good quality is the "Grana Padano" which is made in the southern provinces of Lombardia and which has reached, in the last decades, a quality that's pretty close to the Parmigiano.
Those two brands are both made by a consortium of producers and are protected by a Copyright. How can you recognize the real ones? It's easy: they have branded on their crust the mark, "PARMIGIANO REGGIANO" or "GRANA PADANO". You must find on your piece, if it's not too small, some letters of the brand, otherwise it's likely you're buying bad stuff.
Another advice: consider that in Italy (where it's surely cheaper than in US) a first quality Parmigiano Reggiano costs about $ 10 a pound, so if you are paying it less probably it's something ordinary...