There are not so many CD shops in Florence, and most of them are relatively small. For classical music, La Fenice in via S. Reparata. For Jazz, Twisted in Borgo S. Frediano. For a good selection in all genres, Alberti in Borgo S. Lorenzo (but they recently changed the layout of their shops so I do not know how large a space was left to Cds). Another generalist is Ricordi, via Brunelleschi (on a corner of Piazza della Repubblica).