There is a Telcel vendor on the west side of Aijijic Plaza (Next to Chile Verde)… It is run by two very nice ladies (One speaks perfect English)… I walked in an they set me up with a SIM card and Amigo 200 plan in 15 minute (Sim 129 peso plus 200 peso for month of data). They also handle prepayments to maintain the plan… I now give them 1,000 pesos for 5 months of service (5 x 200)… They give you a receipt with the date to come back before it runs out…
You can choose any monthly prepaid plan and any amount of prepayment you like…