Some typical Thai ingredients can be hard to find here in the states! Here's where to look and what to do if you can't find it:
You can find lemongrass in the herb section of some grocery stores, at a local Asian Market, or online. Use it while it's fresh! The Asian Market near me sells it in sets of two stalks. This recipe only calls for one, but I try to use the second one for another batch of soup within a week because I find that as soon as the lemongrass dries out it loses it's flavor. I've had poor success in freezing it, unfortunately. (Anyone have tips?)
In this recipe, I call for ginger and not galangal. Most Tom Yum recipes call for both, but galangal is another ingredient that can be hard to get your hands on. If you do go to an Asian Market, look there and use half galangal, half ginger in the recipe. Otherwise, don't worry about it for one minute -- the favor of ginger is very similar to galangal (galangal is just a bit more mild), so using all ginger is just fine!
Kaffir lime is in a lot of Thai recipes but I don't include it in the instructions below at all because I've found it's nearly impossible to buy in the states! (That said, it has an amazing aroma, so if you can find it, add 3 leaves to the pot before closing it up, and then remove them before serving). It's also very hard to buy kaffir lime seeds in the United States, but I would love to grow one! I have seen a few companies that will ship a grafted plant, so I might try it.
Last but not least: fish sauce isn’t for everyone (but you can usually find it in the Asian section of your grocery store). If you're not that into it, use Worcestershire sauce instead (it's ok, I have my own trepidations). It's not quite the same, but it will provide some of that Unami flavor that rounds out the broth.
Phew! Thai food is an adventure on it's own! But don't let that stop you -- this soup is delicious, nourishing, and cozy. And this version has been adapted to make it a bit more approachable. Enjoy!