Thai chilis are among the most famous ingredients of Thai cooking and the primary source of the cuisine's infamous heat. These small chilis commonly called bird's-eye peppers, are not the hottest in the world, but they do pack a serious punch. If you can't find these chilis in your local area, there are plenty of substitutes that will heat up your food to Thai chili levels, though the quantity may need to be adjusted to achieve the same heat.
Thai Chili Basics
Thai chilis are about 1 inch long and either red or green. They are commonly cooked whole, after being bruised and broken to allow their heat to flavor whatever food they are cooked with. Thai curry pastes typically incorporate these chilis, including their spicy seeds, to create hot curries; the chilis also are commonplace in the soup tom yum and in the popular condiment nahm prik. If fresh chilis are unavailable, dried Thai chilis are more widely available and lend the same heat when cooking Thai food. When using dried Thai chilis, 1 teaspoon is equivalent to about two Thai chilis.
Serrano peppers are commonly used as a substitute for Thai chilis, though they are not as hot and more peppers are required to obtain the same spice level in a prepared dish. Not just a substitute, serranos have gained favor in Thai cooking and are now being grown in Thailand as well. These 2-inch-long peppers are easy to handle and cut and complement many Thai dishes. They are a thick-fleshed pepper and rarely dried for this reason; find them at your local grocery store.
Jalapeno peppers are widely available and can work in a pinch as a substitute for Thai chilis. Jalapenos are quite variable in their heat level and care should be taken to ensure the spice level before adding too many or too few to a dish. Use jalapenos in the same way as Thai chilis, bruising their skin and breaking their flesh to release the capsicum into the dish you are preparing. Depending on the desired heat, you may have to add as much as double the amount of jalapeno to a dish to achieve the same heat as a Thai chili.
Cayenne peppers are not as widely available fresh, but if you have a source near you, these are the closest in heat level to Thai chilis. Measuring in at 6 inches long, these wiry long peppers are most commonly used for Cajun dishes. A mainstay in sauces and salsas, as well as pickled and dried, these chilis heat up a dish very quickly with just a small amount. Use cayenne chilis as an equal substitution in your recipe for Thai chilis.