If you want that "Thai experience" at home just take some veggies and something spicy (jalapenos, peppers) and brine them in 1 part salt, 3 parts cane sugar, and 1 part fish sauce for about 48 hours.

Meant to say - if you can't find bird chilies, substitute another thin-walled, super hot chili from your market. I've used thinly sliced jalapenos and serranos and while the flavor wan't identical, it was better than nothing!

I wonder if you're thinking about nam pla prik (or nam prik pla). it's thinly sliced or whole Thai bird chilies in fish sauce. There's also prik dong which is thinly sliced or whole Thai bird chilies in rice vinegar. In either case: chilies into a jar, cover with fish sauce or vinegar (a little if you're after chilies, by an inch if you want more liquid for drizzling. Keeps 1-2 weeks in fridge.

Most likely Thai "bird" chilis. They are short, narrow and nasty hot. But they are easy to grow if you live in pepper growing climate and can find starts.