In every place I have worked, which is multiple states, XHHW has always been the standard for stocked AL single conductors, but THN for CU. THHN AL is a available, but needs to be brought in from the distributor. No one seems to know why this is.
Aluminum conductors are typically XHHW. We once did a job where they specified XHHW copper but that wasn't a stock item. I've never seen THHN aluminum.