I just pulled one out of the river last weekend. Big wide piece of driftwood. Cleaned it off and once I figure out how I want to cut it up I'll bake it to sanitize it as described in the earlier posts.
Outside. Mine are too big to fit in the oven, so I don’t bake them. But be prepared for bugs to hatch out if you don’t.
Collect from clean areas away from roadways, agricultural land and waterways. These can be sources if chemical contaminants and such. Avoid spongy, punky or decaying wood.