Apart from the Pisac market [1], there are crafts shops open all week long (10 a.m.–9 p.m. daily) on all the main streets leading from the square, especially Mariscal Castilla, San Francisco, and Bolognesi.
Contemporary art is sold in the first floor of the Café Art Gallery Mullu (Plaza Constitución 352) and in a related gallery a block from the square on San Francisco.
Links:
[1] http://www.moon.com/destinations/peru/the-sacred-valley/pisac/sights/pisac-market