It's called a Tonto Pass. Sells for $6. Use this site to find a location near you. I suggest you pick it up ahead of time or at the ranger station just after turning onto the Barlett Lake Road. Don't punch out the day & time until you arrive.
http://www.fs.fed.us/r3/tonto/tp/vendors.php
Most Big 5 sporting goods, Cabelas, Bass Pro Shop, Sportsman's Warehouse, etc.
AZ Bob