1. Many Russians are afraid of counterfeit vodka. You can certainly buy some vodka with catchy names, like Putinka, but I wouldn't expect good quality. Russians I know typically ask me to buy vodka in duty free stores overseas. Stoli we drink in the US is actually made in Latvia.
2. Russians don't wear them, so tourist shops are your only bet.
3. In large stores on Nevsky. But don't expect it to be cheap.