The best place to buy glass is on the island of Murano, where it has been made since the 13th century. All the main manufacturers have their furnaces and showrooms here, catering to mainstream taste. Some manufacturers also have showrooms in Venice itself.
On Murano, Seguso and Barovier e Toso make glass to traditional designs with good simple lines. You will find similarly attractive pieces in Venice at Paolo Rossi, where they specialize in reproductions of antique glass.
Pauly and Venini both have shops near San Marco. They represent the top end of the market, and some of their designs are extremely attractive.