Ground contact treated wood is easily available, and not expensive. It's usually pine, and pay attention to the treatment solution and strength (there is much variation in what's commercially available - a short net search will tell you what's best of the stuff available in your area). I've short fence lines (< 100 feet in most places), so used 4X4s dropped into a 3 foot hole (I've a post hole digger) with 80lbs of quickrete - that's way overkill for a long fence line at a farm. The round wooden poles, and the galvanized ones, can be driven into non-rocky soil with a FEL, if you have 2 folks working, one on the tractor and one centering the posts (a short piece of 2X6 centered over a post-hole pounder - the manual kind - onto which you lower the FEL works fine). Takes a bit of practice, but once you get going.....you can get in a fence line fairly quickly.