The SF Opera has $30 discounted tickets on sale at 11 am the day of the performance.
Same with the SF Ballet though they stop selling them an hour prior to the performance for some weird reason.
SF Symphony rush tickets are just $20 and you can even check availability by calling 415-503-5577.
Seniors can pick up $20 tickets at Cal Performances two hours before the show.
Limit at all venue is usually two tickets. Of course nothing is guaranteed. Forget about getting tickets for a really hot ticket .