SUBSTITUTIONS:
Galangal - There is really no good substitute for galangal, so first try the international produce aisle of your local grocery store, organic food mart or Asian food store. If it is nowhere to be found, it can be purchased online.
Thai basil leaves – try a farmers market, one of those high-end grocery stores, or Asian food store. If you cannot find Thai basil, you may substitute sweet basil – just add a few more leaves to the recipe as Thai basil is stronger and spicier than traditional basil.
Shrimp paste – I have only seen this ingredient in International/Asian markets…are you seeing a theme here? However, if you cannot find shrimp paste, simply add the equal measurement in fish sauce. Most local grocery stores with an international aisle will carry fish sauce. However, in a pinch you may substitute soy sauce – although, the dish may taste a bit weak.
Tamarind paste – Like the shrimp paste, I have only seen this in International/Asian markets (if you can’t find it in a jar, check the frozen food section). I have seen the pods in my local grocery store, although with its very hard and sticky seeds trying to make them into a paste using a mortar and pestle turned out to be quite the kitchen disaster. As a substitute, I have used ¼ orange juice mixed with 2 tablespoons lime juice, which yielded good results. Simply use a 1:1 measurement as called for in the recipe.