I use both store bought and free wood from the woods. Store wood is generally safer, however I had a really bad run in with some thing called grape wood I got from petsmart. Put a big chunk of it in my 55gal and basically nuked the tank. Caused a nitrite spike, a bacterial bloom, and took out about 20 fish and a lot of $. Since this I tend to soak my wood in a pail and monitor the water quality for a few days before adding it to the tank.
As for as free wood goes, I've not had any incedents with it. Even without boiling. I'm not picky about what I use. I've had no problem using old pine. By old pine I mean the dead weathered branches at the bottom of 100' trees. I run planted tanks though, so slight decay doesn't seem to be a problem. For the most part though, when looking for free wood, I look for dead branches still attached to trees. Or fallen trees that have dead wood raised off the ground. Vines work good, but they float.... alot of free wood floats actually.