I believe this plant used in Thai cooking is leaves from the Kaffir lime tree. You can call local nurseries to see if they're in stock, if not, try the Internet. They cannot tolerate extreme cold so if you plant it outside, you must keep it from frost. My tree is about 5 years old and is close to 6 feet tall. I use the leaves often in Tom Kha Kai (the delicious Thai soup.)

Just to clarify: 'tom' is Thai for soup, while the leaves are makrut (magroot, mah-kroot or however you like to spell it, there is no consistency), Citrus hystrix, usually known in English as kaffir lime.

There have been many discussions in these forums about growing kaffir lime in colder climates, and where you can get it in the US. Go back to the Asian Vegetables page and use the Search window at the foot of the postings, clicking the button for 'Entire Site'.