The superb Erarta Museum of Contemporary Art has several shops in it, including its excellent gift shop, a well-stocked art bookshop and commercially run galleries where art is for sale. Definitely one of the best places to buy contemporary Russian art, as well as creative and unique gifts.
From Vasileostrovskaya metro station, catch bus 6 or 7 to the museum, or trolleybus 10 or 11 from the opposite side of the road.