Long shot is right, Assaesm. I grew up in Pgh and have never found any authentic regional Mexican food there. But if you're going there anyway, maybe give a call to the owners of Las Palmas Carniceria on Brookline Blvd. If it turns out they're from Puebla, MAYBE they'll make some up for you. (412) 388-5090
Casa Oaxaca in DC makes Chiles en Nogada for about a week every year around this time, as an Independence Day special. Is that too far?