Was there last summer, didn't find anything very rare, but a fair few £15 - £40 things, try Grooves on Market St,  Amoeba in the Haight, and Groove Merchant.  Rooky Ricardos shut , not sure if it has reopened.  A number of decent stores in Oakland across the bay, quite accessible by train or ferry, try Hercules records, Alcatraz Ave.