If you feel that a more "underground", alternative style suits you better, then you will prefer this cool shop rather than average high street boutiques. Sanantonio42 is a heaven for vintage lovers: you will find not only street-wear clothing, but also a wide range of vinyls (with soul, r&b, rap and rock music), skateboards, DJ accessories and products for graffiti.
The area around the Tower is packed with tacky souvenir shops. Ignore those and walk to Corso Italia and Borgo Stretto, the two main shopping streets in Pisa, where you will find any kind of Italian fashion shops. The cheapest brands are in Corso Italia and the most elegant boutiques lie in the medieval Borgo Stretto. Two other streets, Via Mercanti and Via dei Rigattieri, boast many excellent shops and all around the city centre you will see dealers who sell old books and paintings. If you are, even vaguely, passionate about antique trade, don't miss the Antique and Handicraft Fair, which takes place every second weekend of the month in Via Santa Maria, Piazza Felice Cavallotti, Via dei Mille, Via Corsica, Piazza dei Cavalieri and Via Ulisse Dini (not in July and August, though). Another must-see is the Mercato delle Vettovaglie, a fruit and food market on the square of the same name, with a centenary history, opened from Monday to Friday.