Now where's the fun in that? Here's a recipe which seems rather authentic:
Obtain the large stomach bag of a sheep, also one of the smaller bags called the king's hood, together with the "pluck" which is the lights (lungs), the liver and the heart. The bags take a great deal of washing. They must be washed first in running cold water, then plunged into boiling water and after that, they must be scraped. Take great care of the bag which is to be filled for if it is damaged it is useless. When you are satisfied it is as clean as you can make it, let it soak in cold salted water overnight. The pluck must also be thoroughly washed; you cook it along with the little bag.
Boil the pluck and the little bag in a large pot with plenty of water, [here comes my very favorite part of the recipe...](leaving the windpipe hanging over the side of the pot as this allows impurities to pass out freely) for about an hour and a half before removing it from the pot and allowing it to cool. Reserve the cooking liquid for later use.
When cold, start preparing the filling by cutting away the windpipe and any gristle and skin. Use only a third of the liver and grate it, then mince the heart, the lights, and the little bag. It may be that you find that the heart and the king's hood are not boiled enough in the hour and a half, and if so, put them back in the pot and boil until tender.
Chop finely one-half pound of beef suet. Toast three handfuls of oatmeal (finely ground oats, or rolled oats; not the "instant" or "quick cooking" oats) on a cookie sheet in the oven.
Then mix all the ingredients - minced lights, grated liver, minced heart, minced king's hood, suet, oatmeal, salt and a good shaking of black pepper. Make this into a soft consistency with the water in which the pluck etc. was boiled.
Place into the stomach bag. Fill only a little over half full as the mixture swells. Sew up the bag with strong thread and the haggis is now ready for cooking.
Use a pot which will easily hold the haggis, and place a plate or trivet in the bottom of the pan. Place the haggis on the trivet, and add water to almost cover the haggis. Bring the water to a boil, and keep it boiling steadily for three hours, pricking occasionally to allow air to escape.
The haggis should be served on a platter without garnish or sauce.
From "Traditional Scots Recipes" by Janet Murray.
As brewcrew pointed out above, the sale of sheep lungs is illegal in the United States (there must be a logical reason for this; our Federal Government would never be arbitrary, would it?), so you'll have to depend on the kindness of any friends you have who happen to be shepherds, if you want the real thing.