Is there a place in the Cancun airport to buy a sim card for an unlocked phone to call within Mexico and to US?

Do I need a power adapter in Mexico? My adapters say North and South America.

<Do I need a power adapter in Mexico?>

No. Same as US.

Mexico is in North America !

I have an unlocked iPhone. I bought a Telcel SIM card (about $4 U.S.) in Mexico to use for local calling. It's a pay-as-you-go type of SIM card. You just buy minutes in advance. When you run out, you can buy more minutes at any Telcel retailer (or OXXO convenience store). There's a Telcel store on every corner in Mexico.

No, you don't need a travel adapter.