Check around online, some websites sell driftwood especially for aquariums. ADA (Aqua Design Amano) has the loveliest pieces of hornwood, I have one and I love the way it looks. It sank straight away, too. No soaking needed. And I'm pretty sure that ZooMed mopani wood is safe.
As said before, rocks are a trickier business because they can alter the PH of the water. I know for sure that "Texas Holey Rock" and limestone are a no-go. Apparently, you can test a rock by pouring vinegar onto it. If it fizzes and bubbles, it's not safe.
In this day and age, I don't trust taking things from the outside - too many pollutants, chemicals and whatnot.