That's a very vague term and one that seldom appears on a bottle. Can you be more specific about what you're looking for? Madiera, Marsala, Malaga? I guess not French, Australian, or California since you said vino.
Around Civic Center, the best selection of unusual stuff is Arlequin.
Arlequin Wine Merchant
384 Hayes Street, San Francisco, CA
Try K&L, at least they know what rancio is.
Valdespino "Solera 1842" VOS Oloroso Jerez (375ml)
This exceptional old Sherry is made from a Solera established in 1842 and the youngest components of the blend are at least 20 years old. It is the most "rancio" of any of the Sherries that we carry, and has an old fashioned intensity that is sure to impress. A small amount of Pedro Ximenez gives the wine balance. This exceptional wine is a perfect partner to strong cheese and paired with a few would make a great dessert. - Gary Westby, K&L