There is a 3 Shop upstairs in the departures check-in hall. But you will also be able to buy the card from 7-11 in the arrivals area.
Just one note, the Peoples Pre-Paid SIM Card is better IMHO as it offers free unlimited IDD to around 20 different countries (only pay for local airtime). It's also available at the 7-11, as are a host of other pre-paid sim packages.