Galangal powder is dried, ground galangal root. Galangal is a member of the ginger family and is used in Thai, Malaysian, Vietnamese, Indonesian, Laotian and Cambodian cooking. Typically fresh galangal root is preferred.
You can purchase the powder in most any Vietnamese grocery store. You can find in many Chinese grocers as well. You can purchase dried, sliced galangal root at Amazon.com: Dried, Sliced Galangal.
If you have a recipe that calls for ground galangal powder you can use equal amounts of one of these ingredients: