The great news is that you can make a buttermilk substitute at home in a couple of different ways and it's unbelievably quick and easy. If you can't get your hands on any buttermilk you can also do a straight swap for standard kefir instead too.
The best way to make your own buttermilk is to mix sour cream and milk. Use 3 parts sour cream with 1 part milk (you can use any type of milk, it even works ok with water). This is the soundest method to use for this recipe as it gives a lovely thick and creamy consistency which is perfect for clinging to the chicken.
You'll also see a lot of recipes telling you to mix lemon juice and milk together. You'll need around 1 tbsp of lemon juice for every 250ml of milk, left for around 15-20 minutes. Whole milk works best. While this does, for most intents and purposes, work, it is essentially making acidic milk rather than giving you a fermented product. It will suffice for this recipe, but the sour cream method gives a more authentic result. If you go down this route you will find that the milk coagulates and you get a lumpy result so you may want to whisk it vigorously before using, or place it in a lidded jar and shake.