My prefered brand is Royal Oak Chef Select restaurant/institutional (marked "Not For Resale") in 40 lb. bags. I can get those for $17.00 a bag and have been quite pleased. As a side note, I find it kind of funny they say Not For Resale yet I'm buying them.
@ KitR - I use the 20lb bags of the Chef Select - I pay $6.00 a bag that is marked "Not for Resale". I love the stuff in my big smoker and grill!