sourdough starters are the do it yourself type of thing, not something you can buy since it is a living organism and requires maintenance, but it is easy to make your own, here is a site for reference. http://www.io.com/~sjohn/sour.htm,
the neat thing about sourdough is that the starters are made from the local yeast colonies that float around in the area you live, thats why san-francisco sourdough tastes different than the sourdough from somewhere else like toronto for example because the yeast cultures are different from area to area. the only thing i can think of is try a bakery for a bit of thier starter but chances are they wont sell because its like a trade secret to them saying thier culture is from france and they dont want someone stealing thier unique taste.
so all in all start your own, easy to do, easy to maintain..
Second mail order from www.sourdo.com. Amazing selection of starter yeast cultures, including San Francisco AND two types of Neapolitan pizza dough. Ed Wood's book Classic Sourdoughs is also excellent.
Alternatively, if you ask a good local sourdough baker very nicely (or deceptively), he/she may give you a pinch of their starter.
They are very easy to make and only take about a month to strengthen enough to be able to raise bread. You start with organic rye flour (which carries better yeasts -- the final product will not taste like rye), organic bread flour and filtered water. There are very good instructions on how to get a starter going in Rose Levy Beranbaum's Bread Bible. You then only need to feed the starter weekly, with bread flour and water.
If you don't want to make your own, you could try buying some off of local bakeries. Woodlot Bakery makes a great sourdough; not sure whether they would sell you some starter.