Structural lumber was originally cut in 2" dimensions at the lumber mill; 2x4, 2x8, 4x8, 6x10. As this industry moved to pine and spruce as the major wood type, the trees were cut down to size "wet" and THEN allowed to dry, thereby decreasing their overall dimensions.
Today, there is no such thing as a true 2x4 or 2x6 or 2x8 etc.
You can start out using a low-grade piece of lumber, but I would think, for the long term, you might want to go with pressure treated.
Very often, at your local "big box" store, you may be able to find remnant pieces of lumber near the "cutting deck", where the store cuts down lumber for their customers. Start there.