In response to the "Why Store Wheat?" letter, most people don't realize that you can easily make your own sourdough culture, from scratch. The air is full of yeasts (most not as quick-acting as the brewer's yeast used in most bread), and it's easily captured. Make a ball of wet flour, and leave it on the counter. Better, leave it in a clean bowl with a mesh cloth or something on top to keep insects out, sitting in a warm place. The next day, add more flour and water and make a bigger ball. Do that for a few days, and eventually you'll see it growing and bubbling thanks to wild yeasts. Voila, one sourdough starter.
Some people keep the flour much wetter, or don't add new flour, and there are some people with specific recommendations for the time it should take (maybe a week or two), but in general, the idea is to get flour wet and leave it out a while. Some people will suggest you use rye flour instead of wheat, as rye has less of various substances that inhibit yeast growth; if you do that, introduce wheat flour to the culture slowly, so it has time to adapt. You'll know your culture is healthy when you feed it new flour and water in the morning and find it's nice and bubbly when that night; if you change flour quickly, like from rye to wheat, or even between different varieties of wheat or methods of grinding, you may find it doesn't get as bubbly as quickly.
Sometimes (lots of the time, even), your new starter won't taste terribly good, or the culture won't be terribly healthy. If that happens, throw it away and try again. Perhaps leave your new flour ball in a different spot, or start it at a different time of year, in hopes of catching different kinds of yeast. Above all, find ways to use sourdough. If it's not used regularly, the culture gets lazy and doesn't behave nicely. I recommend regular use in sourdough pancakes, as well as the occasional (or frequent) batch of bread. - Josh T.