You can obtain another Spiritomb from the Shrouded Ruins during nighttime, after Request #22 is complete in Pokemon Legends: Arceus. It has a chance to be shiny as well!
Pressing X on map screen brings up all locations you can teleport to, which can simplify the amount of button presses or analog movements, if you're hunting for Spiritomb.
Hope this helps!
EDIT: With the new Daybreak update in 1.1.0, there's a fresh method to hunting Alpha Spiritomb in the form of an outbreak! This significantly increases the odds of Alpha Spiritomb's encounter.
