Compressed straw bales from Tractor supply. They last a year easily if you get em up off the ground, and keep it covered or protected from elements. These are far different then regular straw or hay bales. The retaining pressure bands go around the outer edge, instead of across the face, so you never hit them. Instead of shooting into side layered material like on normal bales, its the cross/end sections you shoot at. I'll never buy anything else, its crazy to. I get a bale that is bigger then standard layered foam targets, for $11....and it last just as long.
Just so you know, I shoot enough, I wore out the core of an expensive 3d Rhinehart self healing foam buck in less then 2 months. The straw bale ive had 4 months, and its barely showing wear.