Upvote:1
I go to Mexico City three times per year and have always bought a Virgin Mobile SIM Card in the 7-11 in the airport (I think other 7-11s carry them but I just use the one in the Airport). There are a couple 7-11s in the Juarez Airport and only one of them had the SIM Cards but it was there.
One thing to know is that Mexicans don't call them cards - even in Spanish "Tarjeta" will not communicate - they call it a "Chip" which in Spanish is sort of pronounced like "CHEEP".
You can say "Quiero comprar un chip para el telefono celular; Virgin? Telcel? Movistar?"
I suggest unlocking your phone before you leave the USA - I've used several different Samsung phones and never had a problem.
You can also go to a Telcel store in the city, I had a colleague do that but he waited in line for almost an hour and it was a lot of red tape (they were cheaper on the data and calls though).
All 7-11s sell credit for phone and they all take US Credit Cards.