Galangal powder comes from the galangal root, which is a rhizome similar to ginger. It is sometimes called Laos root or Thai ginger because of its frequent use in Laotian and Thai cuisine. If you can't find any galangal powder, you can complete your recipe with a substitute.
Fresh or Frozen Galangal
If you can't get hold of galangal powder, fresh or frozen galangal root is the best substitute. You should be able to find galangal root in a well-stocked Asian grocery store. Replace every 1 teaspoon of galangal powder with 1 inch of the fresh root. Fresh galangal can be quite tough and woody, so you may need to cut it into shards before grinding it to a paste in a spice grinder.
Ginger is in the same plant family as galangal, so it makes a reasonable substitute for galangal powder. It doesn't have quite the same citrusy notes, but it has a similar flavor profile. You can use either fresh or dried ginger, but fresh is a better choice because it will produce a richer and more complex flavor. Be aware that ginger is slightly more pungent than galangal, so use slightly less.
Although it won't replicate the flavor of galangal powder, either black or white pepper makes a reasonable substitute. It will add a slightly sharp and spicy taste that is similar to the notes in galangal. While it's not the ideal substitute, it's an ingredient you're likely to have on hand and it won't leave your dish tasting bland. Add it according to taste, but start with 1/2 teaspoon for every 1 teaspoon of galangal powder you're replacing.
Mace and Cinnamon
The combination of mace and cinnamon makes a surprisingly good substitute for galangal powder. The cinnamon gives a touch of sweetness while the mace adds sharpness and complexity. While it won't taste identical to galangal powder, it may work. For every 1 teaspoon of galangal powder you're replacing, add 1/2 teaspoon of cinnamon and 1/2 teaspoon of mace.