In Puerto Vallarta we went to several places to find a SIM card but they turned out to be only available at the ubiquitous OXXO, the 7/11 of Mexico.
¿Está tu celular bloqueado? First question is your cell phone locked? If it is, you can’t get a new SIM.
No one in OXXO spoke more than “un poquito de ingles”. Acquiring a ‘tarjeta SIM’ for Al’s ‘movil’ was fairly easy. Now we had a phone number but no one seemed to understand the word ‘data’.
“Por un mes” (for a month) got a response though, and the clerk took us to the back, to the freezer of all places, and pointed at a card posted there.
All was now clear. The card listed ‘paquetes’ of data (which it called ‘megas’ or ‘dates’, NOT ‘data’) along with lengths of time on the far right in días (days). Al opted for 2 1/2 G data (listed as 2,500 MB) and 33 days for 300 pesos, approximately 22 dollars CDN in 2019 prices.
The SIM card and ‘megas’ were paid for separately. The clerk installed the SIM card for us and the data added. The clerk pointed at a number which Al assumed was the number he had to phone to activate the card.
We retreated to a quiet part of the store, and Al phoned the indicated number, but kept getting a message saying ‘el numero que marca esta occupado’ (the number dialed is busy) over and over again.
Back we went and questioned the clerk. It turned out that the number that Al was repeatedly calling was his own – no wonder it was always busy. Apparently the card was automatically activated once it was purchased and so was the data.
And we were good to go!
Albatz Travel Adventures 
We did this on our Mexico tour last summer, but we used Telcel, who have shops all over the place. Really quick to get sorted in the Guadalajara shop, helpful staff and a cheap deal. Recommended!
Good to know – I posted this for a friend who is going to Mexico for two weeks and her local provider wanted to charge $96 for a two week in Mexico connection. I’ll pass this on.
PS. I seem to remember going to Telcel and getting rather confused -probably wrong but it seemed to us that they wanted to sign us up for a long term deal, and when we only wanted a month they sent us to the ubiquitous OXXO…
They were fine for a 30-day deal…and if I remember correctly you can extend after 30 days without going back to the shop. It was definitely easy and cheap.