One way to score some free wood is to get old shipping pallets, which are easy to find for free. A lot of pallets (most, it seems) are made of treated wood but some are heat treated instead. You can easily identify which are heat treated because they will be stamped "HT" like this:
New pallets have to be stamped with a country code (the ones above were made in the US and the one below in Mexico) followed by a number identifying the plant followed by either "HT" for heat treated, "DB" for debarked (this is also a safe type of pallet to use) or "MB" for chemically treated with methyl bromide. It is the MB's that you want to avoid.
I found two of those pallets at my local feed store. Most of the pallets in their stack were not marked so I grabbed the only two marked "HT". Later the guy at the feed store told me that the heat treated ones are the only ones that the shipping companies will take back. Feed stores are a good place to look for pallets, as is any store that receives shipments of heavy items. The shipments are unloaded off the trucks by forklift, which requires a pallet on bottom. The pallets I used to make my compost bin came from a pool supply store. (Pool supply stores carry a lot of heavy 5-gallon buckets of chemicals.) Other places to check might be paint stores, grocery stores, and hardware stores.
I always ask permission before taking pallets even if they are next to the dumpster. Most places are happy to get rid of them. I also see pallets listed in the "free" section of Craigslist all the time.
Where do you guys find pallets? Do you look for the heat treated or debarked ones? I'm going to need quite a few for my veggie bed project so I'm on the prowl.