"The correct question is not where but why," chuckl deadpanned when travertine6 asked about places to buy good box wines in the Bay Area. Curiosity is one reason to, admits the OP, along with the appeal of having wine "on tap" at home.
Even New York Times wine critic Eric Asimov recently wrote about the attractions of box wine: It "resolves this problem of oxidation by eliminating space for air to occupy. Wine can stay fresh for weeks once it has been opened." Asimov noted an improvement in quality over past years and recommended reds over whites, which he says fell flat in tastings.
Locally, many chain stores and supermarkets carry box wine, but hounds suggest a few spots that sell higher-quality vino. Arlequin Wine Merchant in Hayes Valley offers French and German box wines, though alfairfax admits he "was not bowled over" by the German Riesling he tried. Local chain K&L carries direct imports that aren't available elsewhere, and Robert Lauriston recommends Pierre Plouzeau Chinon at $50 for 5 liters, or about half of what it would cost if it were bottled.
If you're going the box wine route, Windy suggests seeking out upscale grocers like Glen Park's Canyon Market and Whole Foods, which might be more thoughtful in their selection and handling than big box retailers that sell jug-quality wines. Franzia is a fate we wouldn't wish on anyone.
Arlequin Wine Merchant [Hayes Valley]
384-A Hayes Street, San Francisco
K&L Wine Merchants [SOMA]
638 Fourth Street, San Francisco
Canyon Market [Glen Park]
2815 Diamond Street, San Francisco
Whole Foods Market [SOMA]
399 Fourth Street, San Francisco