Reclaimed wood is gaining more and more importance in furniture, products, construction and interiors, for DIY as well as commercial enterprises. Old buildings, barns, wooden decks, old furniture, boats or wooden crates are common sources of wood for product design. You can contact demolition companies to find reclaimed wood.
Some types of reclaimed wood, for example railway ties, have toxicity issues because of lead containing paints.
Painted wood can be easily be tested for lead.
Examples on Superuse.org
Examples on Harvestmap.org