Occasionally I find a thing or two at yard sales. The (Habitat for Humanity) ReStore is a place to check out. Go on their website for a list of stores in your area.
I got a great price on laminate flooring at www.builddirect.com a few years ago. I had to buy 350 sq. ft. more than what I needed (I had to buy a full pallet), but I still saved about $1,000 (even after paying the extra shipping and handling) over what I would have spent at Lowe's buying just the # of sq. ft. I needed to do 6 rooms and a hallway. I'll probably use the extra 350 sq. ft. downstairs or sell it.
Build Direct also sells other kinds of flooring, ceramic, roofing materials, decking, siding, counter tops, sinks, etc.
Like Scuba Dave, I've also gotten some good deals from people on Craig's list. It's kind of a hit-and-miss thing. Sometimes you'll find a great deal on there and other times you'll find nothing.