Reclaimed lumber can come from timbers and decking rescued from old barns, factories and warehouses although some companies use wood from less traditional structures such as boxcars, coal mines and wine barrels. Another type of reclaimed lumber is blow down wood, from storms and other natural causes. This wood  must be thoroughly dried in order to avoid warping and inspected for insects. A third type are the logs felled by the early logging companies in the southeastern US, these logs were lost in the rivers on their way to the mills. Goodwin River-Recovered® wood is an example of this type.

Salvaged wood is actually easier to locate than in the past because now old structures are often deconstructed, rather than demolished.
Keep in mind is that salvaged woods from modern structures may have been treated or finished, at some point in their life, with petrochemical-based products and/or lead paint. Older wood is less likely to have been finished with toxic products.

Find lumber that has been salvaged domestically, rather than shipped in from Southeast Asia or South America for the greener choice.

Salvaged Lumber DIY

Shipyards: Shipping crates are often made of high-quality timber.
Garage Sales, Thrift Stores: Old furniture, frames, headboards, and other furniture can often be re-purposed.
Structures Set for Demolition: This is prime real estate in the reclaimed wood world. Goodwin’s Legacy Heart Pine has been salvaged from Industrial Revolution era warehouses, and other old structures.
Architectural Salvage Yards:
Online: Craig’s list.org and Freecycle.org are excellent places to search for reclaimable wood.

Thanks to wikipedia.org