People often ask me where I get my reclaimed wood materials. One good source is Build It Green out in Queens. It works through donations of materials, so it can be hit and miss, but the staff is friendly and if you wanted to build a reclaimed floor joist shelf, say, I'm sure you could call ahead (with your measurements!) and ask if they have a suitable piece in stock. It is also just a great place to browse, they get some crazy stuff.