I usually go to my LHBS. It seems Northern Brewer's prices are hit or miss. For example, their Belgian Candi Sugar is 150% MORE than my LHBS, and their grains are about $1 to $2 per pound more than my LHBS. However, their yeasts are priced nice. Their draft towers are an outstanding value too.
I did just run across the thread on Brewmaster's Warehouse. I'll probably use them as much as possible from now on (when my LHBS is out).