Besides Ebay, BrickLink, TRU, Target, etc, etc don't forget to check out your local consignment shop! I found a huge cache of really nice pieces (there were a few BrickBlocks and Mega-Blocks mixed in...) for a great bargain, IMO. About 14lbs for $7.00.
For those of you who don't know...A Consignment Shop is a store where folks bring in old clothes, toys, video, DVD's, shelves, books - pretty much anything one can think of to try to sell them. Rather than holding a year-round sale through your home, the Shop holds the stuff, then when it sells, receives a commission of the sale.
I told the lady to call me right up if she ever gets any other Lego in!