Kaffir Lime Leaves: These are the actual leaves from the kaffir lime tree, and are very common in Thai cooking. You can purchase them fresh or dried. Fresh kaffir lime leaves can be frozen for longer shelf-life, and are my personal preference for this soup. You can also try dried kaffir lime leaves.
Galangal: This is a root closely related to ginger and turmeric. Dried galangal is inferior to fresh, but is easier to source and still works well. Just make sure you allow some time to soak them in water before starting your soup. You can also substitute fresh ginger for the galangal, although it's not authentic and won't have the same flavor.
Lemongrass: Lemongrass comes in stalks, and features a lemony scent and flavor. It’s common these days at many supermarkets, and is found with the other fresh herbs. It also freezes really well, so if you buy more than you need just toss the excess in a freezer bag for next time.
Thai Bird’s Eye Chile: These small, spicy Thai chiles are the authentic ones to use in this tom yum gai recipe. They’re about 10 times hotter than a jalapeño but half the heat of a habanero, and are usually 1 to 1 ½-inches in length and either red or green. I’ve found frozen bird's eye chiles at my local Asian market and used those for this recipe, but have also gotten tiny fresh ones from a local farm before. Many well-stocked supermarkets will also carry them fresh. Be sure to wear food safe gloves before handling these chiles.
Fish Sauce: This is very common in several different Southeast Asian cuisines including Thai and Vietnamese. Fish sauce is made with anchovies and salt, and although its odor is very strong, the flavor is key to making authentic Thai hot and sour soup.
Nam Prik Pao: Also known as Thai chile paste, nam prik pao is available in jars in the Asian aisle of many supermarkets, in Asian markets, and online.
