Lentils come in different colors and the three most common are green, red, and brown lentils. While you can use any kind of lentils in this soup, I recommend using brown or green, if you want the lentil to keep its shape. Red lentils tend to become softer and kind of dissolve into the soup, which is great if you want a smoother, thicker base to your soup. Dry lentils come in a bag in your grocery store where the dried beans are, so they should be easy to find.