in my opinion, locally is the best way to go, only because you can actually see what you are getting, know how big it is, and see all sides of it, so you can plan your scape accordingly. i rarely purchase centerpieces, driftwood or otherwise, online unless i know exactly what i am going to get. often, that also includes an increased price hike, but for me the pricetag is sometimes worth it because its exactly what i want.
that being said, the forums here is a pretty decent place to locate hardscaping equipment. people post pics of the driftwood and rocks they have for sale, and at reasonable price. just last week i saw a post in the trading/sale section offering seiryu stone and manzanita... you might want to start your search there...