If you burn wood check at small sawmills in your area to see if they cut railroad ties. At least locally, those who do will let the cut off ends be hauled off free. They are either oak or popular and usually about 6" x 8" and from about 8" to 20" in length. They split fairly well by hand and work very work very well on a hydraulic splitter due to a flat bottom. The larger ones would be 'all nighters'.