The Subways in Sacramento have sour dough but I believe it to be a far-flung cousin. Much of what is labeled sourdough is nothing like the original, but Boudin and Panera are good substitutes.
Sourdough in Seattle is pretty weak IMHO. It’s widely available but not ubiquitous (eg, not in Subway, not standard in many sandwich shops, not very sour, etc). I grew up around the Bay Area, and the sourdough there is much better than Seattle.
Weird: I live 20-ish miles south of you and hardly ever see really good, honest sourdough. Certainly not from local grocers. I also lived in San Jose for 14 years, and the grocers up there didn’t have good sourdough either Where do you get it, from a bakery? I’m going to be up there tomorrow, I’ve love to get some.
I used to LOVE sourdough bread, but then everyone started making it and most of them, rather than cultivate a proper sour starter, just put citric acid in the dough to make it taste sour. It’s a cheap trick and it’s not the same.
Since I have never consciously had something called “sourdough bread” I can’t really tell, but the looks and description are similar to Spanish panes de pueblo, which is a whole range; crunchy, golden crust, big-eyed interior. The bakery in my village has a dozen different-shaped kinds. The little breads we get at our regular restaurant here in Barcelona are that style as well. I hate most kinds of bread but love these.
Now I’m hungry 
God it’s EVERYWHERE in London, barely a day seems to go by without some artisan bakery opening up under a railway arch round here. I was trying to by some simple soft white rolls for burgers the other day in my local foodie market and practically all I could get was sourdough (or rye, or a million other spongy objects which won’t work with a burger).
I ended up having to track down a chain supermarket.
I wasn’t aware of its Californian origins, I just put it down to artisan bakers reviving old wheat flours in this low GI and wheat intolerant world. Sourdough has been used in Europe for several 1000 years.
In New Hampshire, my supermarket bakery makes it fresh every day.
I’ve never seen it here in Tel Aviv. I don’t think there’s a word for it in Hebrew.
You can get sourdough in a working class, suburban supermarket here in south east Queensland, Australia
I don’t like it.
Pretty easy to get here. Probably wouldn’t find it in small to medium grocery shops but all bakeries and supermarkets would make/stock it.
This is a bakery a 3 min walk from my office.
http://www.bretzel.ie/Sourdough-Bread/View-all-products.html
Korea. Rarer than virgin mink.
Detroit area.
Uncommon
In St. Louis, Companion Bread delivers daily to both the major grocery chains (Dierberg’s and Schnuck’s), and sourdough is one of their major products. So, fairly easy to come by.
Of course, Companion, St. Louis Bread Co. (= Panera), and Trader Joe’s sourdough is only a wan and pallid copy of real San Francisco sourdough. Sort of like the difference between eating a croissant in the US, and then going to Paris and discovering “oh, that is what a croissant is supposed to be like!”
ISTR getting some decent sourdough bread at Pike Place. But generally the sourdough in supermarkets (which is ubiquitous) is pretty weak. I loved sourdough when I was growing up in SoCal. But now I want more choices. How about a nice Italian loaf? Where’s the damned pumpernickel? It seems the choices up here are sourdough, more sourdough, sourdough, or some sort of ‘artisan’ bread. Supermarkets have no imagination.
Have a mint? No, really, I insist. 
While this is true, so is this:
Finding sourdough on the label is super easy in Chicago. Finding “real” sourdough bread inside is a little bit harder. Not anywhere near close to impossible, but harder than I’d expect, it being a big metropolitan melting pot.
I live just outside of DC. Around here, most any grocery store will have a store brand of sourdough in big crusty loaves in their bakery and one or two major national brands of sliced sandwich bread (Pepperidge Farms and the like) in the bread aisle. I’ve never seen a sandwich shop around here that has sourdough.
I’ve only rarely seen it offered in delis and sandwich shops here in NE Ohio.
This is my number one complaint about living on the east coast. I grew up in Sacramento, and sourdough was always our default bread. If you got a sandwich, it would be one sourdough unless you asked otherwise. If you were asked to stop by the store for a loaf of bread, they meant sourdough bread. Breakfast growing up was usually a couple slices of sourdough with butter, olive oil or cheese. Supermarket bread aisles are probably 1/3 sourdough. My favorite was Alfredo of Santa Cruz, which makes a delicious sourdough with full cloves of garlic and sun dried tomatoes.
When I arrived in DC, my local supermarket only had that horrible Pepperidge Farms “soft” sourdough. To my thinking, it is an abomination. Sourdough should have a glossy crust and have some chew to it. A proper sourdough fights back a little bit. Other than that…nothing. Delis don’t have it for their sandwiches, which is just baffling to me. Yes, I can get if I look for it and are willing to pay four bucks for a loaf of bread. But that’s the point- I’m not used to having to go out of my way to get sourdough. I probably ate sourdough every day back home. Now that place in my life is empty.
I never made the connection that it really was a Northern California phenomena. I never realized how deeply that place runs in my veins.
Since I live in Oakland (and work in San Francisco), I pretty much trip over loaves of it when I walk down the street.
In the part of Germany where I live, this kind of sourdough bread (German Wiki link) is the default kind of bread, which means that it’s a staple of almost every household. I like it so much that the thought of living somewhere where it’s not available is almost frightening.