I've bought the chunks from the Depot. Pretty good deal here.I've also bought chips which I wrap in foil and poke holes in it. A suggestion from one of the guys here.The chips last longer.
I also put the word out to my buddys in the country.every time they take a hardwood or fruitwood down they drop off limbs 3-4" dia. and 4-6' long. Those prices are the best free.