Shouldn't be any problem as the new trucks sold In Alaska require it. My Chevy truck uses about a gallon per 1,000 miles so I can fill up here in south Florida and easily make it to Fairbanks. Plus I tend to carry a jug, 2 1/2 gallons, with me on trips.
In 2011 I had trouble finding it in Prince George. Canadian Tire was out, another large parts warehouse was out except for 100 liter containers. Finally found some at a Chevrolet dealer. That was the first year for DEF so I sure there is more available now. Fill up before leaving the states and you should be good all the way to Alaska.
I buy mine at NAPA. Around here when I catch it on sale, it is in the $11 a jug range. Last summer in Gunnison Colorado I paid about $25 for a case of two jugs. Our Wal Mart also sells DEF in the motor oil section.
Enjoy your trip: we're leaving the Cincinnati area for Alaska on June 15th. Going to carry a couple jugs of DEF as well. Just in case. We, too, use about a gallon every 1000 miles, and a five gallon tank should easily get us to Fairbanks.