Pesos, from an ATM at a bank or CUN.
CUN...you mean in Cancun airport...or in city?
If CUN APO, where upon arrival side?
Good exchange? Transaction fees?
Like Chunky said, In the airport.
There are safe, convenient ATM machines just to the left as you come through the glass door after customs in Terminal 3. The car rental counters will be directly in front of you and the gauntlet of Time Share people to the right, between you and the exit to transportation.
Assuming you are coming in from JAX on an American airline, you will most likely be in terminal 3.
The exchange rate at ATM is almost always better than at a cambio exchange booth. After all, they are there to make a profit on tourists.
Transaction costs vary with banks.