Even though I can get lemongrass locally at Far East shops, I usually buy "Gourmet Garden" lemon grass in a tube imported from Australia. This product is usually found in the fresh produce section of your grocery store. They have a website where you can find the list of major grocery stores in each state that carry it.
www.gourmetgarden.com.au
Instructions on the tube say "Just squeeze straight from the tube. 3/4 tsps = 3/4 tsp of fresh lemon grass or 1/2 tsp of dried lemon grass."