Galangal is a common ingredient in Thai, Malaysian, and other Southeast Asian cuisine. It looks like ginger root, but harder and with a smooth, shiny exterior. It has a bit of a piney taste with a citrus aroma and it’s fantastic in soups and curries.
There’s really no good substitute for galangal. I haven’t seen it at too many general grocery stores but many gourmet/organic grocery stores have it, and any decent Asian food store should carry it. If it’s nowhere to be found, it can be purchased online.