I've gotten sourdough starters from health food stores and from gourmet
grocery stores. It comes dehydrated in a foil package, and there are
several different varieties available. Or you can make your own.
The last I bought was from a store in Asheville, NC which was originally
called Dinner for the Earth. When they built a much larger store I think
they changed the name, and I can't remember it off-hand.
I'm not an expert on sourdoughs (although I've done them a few times and
can usually get salt risen bread to do---but not in an abm). I understand
from my reading that "true" sourdough breads do not use commercial yeasts
to rise. That is, one doesn't have to add Red Star, etc. to the recipe
along with the starter. The starter in these types of bread do the "rising
work"
all by themselves.
Then there's the type of sourdough bread that uses a starter (mostly for
flavor) and commercial yeast (I presume for a more predictable rising
time). There's an excellent recipe in Martha Rose Shulman's book
"Mediterranean Light"
both for making a couple of starters and then using starter plus commercial
yeast in breads. One then saves part of the dough to use for the next
batch of starter. I've kept this type of starter (which does have
commercial yeast in it) going for a couple of years or more in my 'frig. I
never feed it and I only use it maybe 6-8 times a year (as I do most of my
breads on my abm). Although the bread I've made from this has a wonderful
flavor (a whole wheat and bran bread with a little white flour), it isn't
as sour as other sour dough breads I've had.
virginia@dnet.net