There is a solid chance of spawns at boar stones. It may never be as nice as the ensure 5+ the first time, but revisits often have several around it for me. Rarely, nothing after the first go.

The best bulk farm for boar I have done to date was simply to build on the edge of the plains. The plains guys play whack a mole with the deer, boar, lizards, and now night spawns of dwarf, skeleton, and whatever else wanders in. You do have to be home at night for this to really pour on the free loot, but all you have to do is pick it up.