I live near San Francisco and I am in love with the local sourdough bread. It is very expensive and I'd like to bake my own bread. I am wondering though, is it possible to make sourdough starter from regular breadmachine yeast?
Yes you can create sourdough from regular yeast, but it's not necessary. Ordinary flour comes with a trace of yeast. In the San Francisco area, that yeast is famed. Just mix flour and water 50/50 to make a batter-like mixture and leave it in a jar on the counter. Put a lightly fitting lid on the jar.
Add more flour and water every 12h (give or take). You should be doubling the initial amount each time, so 1 teaspoon, 2 teaspoons, 4...
After a couple of days you'll see bubbles developing in the sourdough. Once you're at that stage, you can reduce the amount of care. 'Refresh' the sourdough once a week: mix, take out 1/2 of the sourdough, add the same amount back (50/50 flour/water as before), mix. According to the Wikipedia, this can be kept at room temperature.
Sourdough is a mixture of flour, water, yeast and bacteria. When unattended for some time, 'water' forms at the surface. This liquid contains some alcohol that adds to the flavor of the dough. It's part of the sourdough and should be mixed in when refreshing the dough.
You cannot produce sourdough with just bread machine yeast; it lacks the lactobacillus bacteria in sourdough starter, which produce the lactic acid that sours the bread. However, you can use bread machine yeast and sourdough starter together to produce a dough with sourdough flavor, but which a bread machine can handle (see my answer to "bread machine sourdough").
Fortunately, sourdough starter is dead easy to obtain, and is so easy to care for that it makes those pet Sea Monkeys look high maintenance! As BaffledCook suggests, you can prepare your own sourdough starter from scratch. You can also ask bakeries if they will share a little of their starter. Finally, sourdough starter is available by mail order from a variety of sources, including the King Arthur flour online catalog.
Edit: my current recommendation for mail-order starters is Breadtopia's. I recently used some after losing my starter, and it is by far the fastest I've seen. Rise times are just 2/3 what most recipes call for (factoring in temperature), and it creates an airier bread.
The short answer: bread machine yeast is not the right way to start sourdough. Use the natural yeast present in the Bay Area air.
The long answer:
Since you live near here (I live in SF), starting a sourdough couldn't be easier if you have a little patience. I have a sourdough we call "H.P.", as in "Lovecraft", because is it a big, scary, sticky white shoggoth. H.P. has been going for 6 years and has produced countless batches of bread, bagels, pizza dough, pancakes, and waffles.
I'll tell you how H.P. started.
First, buy some unbleached, low-heat milled organic flour milled on the West Coast. Bob's Red Mill, Arrowhead Mills, and Butte Creek Mill all work for this. Guisto's is the best, but they don't sell to home cooks. Second, mix 1.5 cups with 1.5 cups of unchlorinated, filtered water (see below). Leave this out on the counter, at room temperature, for 3 days.
After 3 days, throw away half the goo and add 3/4 cup flour and 3/4 cup water. Leave out for another 2 days. Repeat for another 2 days. Then split and mix again, put in a jar with a loose lid (a mason jar not screwed all the way down works well) and place in the fridge. Leave for 2 weeks.
After this, you should have a sourdough. Split it and replentish it with flour and water every 3-4 weeks minimum, or once a week maximum (unless you're going to get into serious multi-stage production baking). If you find that your sourdough doesn't have much rise, add 1/8 tsp commercial baking yeast (any kind) to it once to improve the yeast strain.
A note about water: most Bay Area tap water has chloramine in it, which improves the water quality but will kill your sourdough. You need to use purified unchlorinated water.
Recommended books on Bay Area sourdough:
Sunset Magazine also has a recipe for making sourdough in a bread machine. Since it appears to rely on yogurt for the bacterial culture, I don't think much of it, but try it if you dare: http://www.myrecipes.com/recipe/sourdough-starter-10000001151410/
Since you are impressed with some local strains of sour dough (for good reason) you might also try to reconstitute one of those local cultures by starting with some of the actual bread, You can do this using baked bread as not ALL of the culture is normally killed during baking, but you may want to try a few parallel starts as not all starts will take off.
Start by getting some of the desired sourdough bread and cut it into cubes about 1 inch, no crust. You will need 7 cubes per attempt.
Use these cubes with @BaffledCooks directions, in place of the flour for the first 3 days of the process (1 cube, 2 cubes, 4 cubes, then switch to flour).
On day 4 you should be able to tell which samples are taking off by the bubbling action in the sample. From there you now have duplicated the culture at your favorite bakery. Of course other ingredients bring flavors to the mix, but with this you should be able to get very close to your favorite sourdough.