Tickets for most events can be bought at ticket offices (teatralnaya kassa), of which there are many in the city centre. Of course every venue sells its own tickets, and with some you can buy via the internet.

e-Tickets
Interested in attending a concert, but you don’t have time to leave your hotel, house or office to buy a ticket? Well there are a number of online services that allow you to reserve and/or buy tickets online and most services offer delivery anywhere in the city for an additional fee. Tickets that are reserved need to be purchased within three days or the reservation expires. These services are very useful for foreigners planning trips to St. Petersburg, because they can order opera and ballet tickets well in advance.
Kassir.ru (703 40 40) – This site has the most comprehensive listing available, but is only available in Russian. Payment options include cash (when picking up the tickets or if delivered) or credit card (MC/V).
Bileter.ru (380 80 50) – The only comprehensive site that offers their event listing in English. Payment options include cash (when picking up the tickets or if delivered) or credit card (MC/V).