HD and Lowes should have 1, 1.5 or 2in readily available, Menards too, although I've never been in 1.
Might buy 2 different thickness, depending on exactly where you plan to put it.
2in is probably cheapest by the thickness/SqFt, but not by much. 2 layers of 1in would provide the same amount, but w twice as much cutting. .. .
Pink, blue or green. Just make sure its XPS (extruded polystyrene) not EPS (typical styro cooler/cup white beads, expanded polystyrene)