Liquor, wine and beer are available on almost every street corner kiosk, especially around Metros and train stations, in most commercial shops and "state shops". Many kiosks and commercial shops are open 24 hours a day. See FOOD STORES - OPEN 24 HOUR FOOD STORES.
Refilling of old bottles or "changing labels" is not an unknown practice among kiosk owners. Do not buy bottles with damaged labels. Look for tamper- resistant tops on bottles such as "Stolichnaya" or "Absolut", "Smirnoff". You will find many unrecognizable brands of scotch, creams, innumerable imported vodkas, amaretto, liquors of kiwi, bananas, etc. and other throat-numbing "spirits" (which may actually do some harm).
The best selection of imported name brands of liquor and beer is often found in the privately owned grocery stores and super-markets that have sprung up throughout St. Petersburg over the past few years. A "best buy" still is "Stolichnaya Vodka" (check the cap and label before buying): be aware that "Stolichnaya" is produced in many different distilleries and can vary in quality and price. More rubles means less rot gut. Russian beer is an acquired taste (see BEER), but Russian champagne is quite good and very reasonable.