We have an old wood reclamation company in our northern california area. One of our clients, a young couple, have wanted to do their landscape construction with as much recycled wood or certified sustainably harvested wood as possible; our designs have taken that into account.
I mention this because it is hard for me to picture the average person at the lumber yards around here even knowing about that company, much less recommending it - though I might be wrong about that and it could be useful to ask.
I have a link for it, not that it would help you get old wood, Swimpy, but that it might give you some hint on how to find such places in your part of the country.
This article is primarily about redwoods, but a company mentioned in it does handle other old growth timbers for recycling use.
http://www.northcoastjournal.com/012705/cover0127.html