Be very careful though. Areas sprayed with pesticides may get it on wood which will leach into the tank. I would also be careful of what type of wood it is, some trees like pine are toxic to fish due to the sap.It's a bit tougher, but you can find your own drift wood and there are ways to make sure you aren't introducing any parasites or issues. I haven't done it, but I know people who have collected their own driftwood for free successfully.
You can also buy some off etsy or other sites online