I use kiln-dried pine, it helps bake out some of the pine oils. I have found it cheap at several locations, but the best in my area is Mills Fleet Farm. If you buy it by the bale, it is $3.90 a bale. If you buy a half pallet (20 bales) it is $3.75/bale. If you buy a full pallet (40 bales), I believe it drops to $3.40/bale. I go through approx. 3 - 3 1/2 bales a week.
Just a side note: newspaper print is not toxic.