I have a recipe that calls for 16 ounces of buttermilk...but don't have any. How can I make buttermilk? What is a substitute for buttermilk?
In recipes, buttermilk brings liquid, some milk solids, some acid, and if it is not made from skim milk (it is a cultured product), some fat. In a pinch, you can substitute approximately 1 cup milk with 1 tbl lemon juice or vinegar.
For information on making your own cultured buttermilk, see this article from Serious Eats. It is made from milk and an active culture starter from commercial or previous buttermilk batches, much akin to making yogurt.
Edit: As hinted in Fisher's answer, for some applications, particularly quick breads and muffins, buttermilk can also be substituted one to one with yogurt or sour cream.
You can also make your own non-cultured buttermilk. Buttermilk was originally the product of butter production. If you follow instructions to make butter (agitating heavy cream, recommended at least 35% milkfat content until the solids & liquids separate--this can be done via electric mixer or even with a marble in a jar), you will have not just butter but also buttermilk. The buttermilk that is made this way is not tangy like cultured buttermilk, but perfectly good for baking--and even drinking.
Traditional old-fashioned buttermilk used fresh, raw cream that was allowed to culture & sour (raw cream contains the right buggies for this and when done in an environment (e.g. a diary farm that had been processing cultured dairy for a time) rife with the cultures it would sour more quickly & evenly. Most folks nowadays use a live lactic-acid producing culture to control the flavor & production. You can also use yogurt or culture buttermilk as a starter for your cultured butter. After you have cultured your cream, you follow the basic butter making steps and wind up with cultured butter and tangy cultured buttermilk.
While various recipes are available online, I have used Ricki the Cheese Queen's recipe for cultured butter: http://www.cheesemaking.com/Butter.html with good results.
It would also be fun to try the jar & marble method (great for kids, too): http://www.ehow.com/how_5678906_make-butter-baby-food-jars.html
Or, yogurt is a good buttermilk substitute. You can use 3/4 cup of yogurt mixed with 1/4 cup milk or water to get the consistency right.