Jason beat me to it. Though many Bay Area retailers also stock some selection, there are three standard established US importers of artisanal German wines, and the smallest of the three, Dee Vine Wines, is local (Pier 19 in San Francisco) and happens to be a major local retailer too, and focal point of much Bay Area German riesling enthusiasm. DVW imports and sells other wines, but German rieslings are the strength. I've dealt with DVW happily for many years. The firm organizes tastings (so get on the mailing list) and imports the outstanding artisanal producers like von Kesselstatt, Loosen, Egon Müller, von Othegraven, J. J. Prüm, Josef Rosch, and St. Urbanshof in the Mosel-Saar-Ruwer appelations; Hermann Dönnhoff in the Nahe; Peter Jakob Kühn, Prinz, and Robert Weil in the Rheingau. These producers can also offer excellent values (unless talked up by some fashionable critic, which always hurts value).
Practical personnel problems a few years ago on DVW's retail side have been fixed, I'm told, and who knows, the Web site may even work for online orders now (DVW's Web site has been notorious among the regular customers. Don't judge the firm by its Web site, judge by the fact that most Bay Area wine geeks seriously interested in German rieslings buy there.)
Dee Vine Wines
19 Pier 45, San Francisco, CA 94133
Dee Vine wine is great. So is K&L. Premier Cru in Emeryville has some nice values from time to time. Bill Mayer imports riesling and has a tie to Valley View Wines, which has some connection to Napa.
Also, I like William Cross Wine Merchants on Polk, although I've only been there once. They have a great selection of wines. I don't recall their riesling selection but I'm guessing it's pretty good. They carry Franken silvaner, which is rare.
1011 University Ave, Berkeley, CA
There is a German Riesling festival in the Bay Area, used to promote the wines. I think it's connected with Destination Riesling. K & L and Blackwell's on Geary participate. I know in the case of Blackwell's they carry am extensive, and fairly unique selection of German wines and offer tasting events year round.
Here's an idea of their current selection which is unfortunately a little heavy on the Prum:
You can also check mail order outlets like Wine Library, who have had extensive stock in the past.