Menards (at least mine) now carries it in 3.5"x15x8?" batt packs, about $50. Not cheap.
However, 3" is too thick for the ceiling. Perfect for the walls.
You do NOT want to try to slit a batt in half (1.75") the stuff is VERY tough to cut. With patience, I did get one bat slit. But that was the only one.
I've got some of the home depot stuff ordered to finish the back of my van. supposed to arrive Friday. We'll see how it goes.