There are some "tour agencies" that buy bulk tickets and then sell them, either at a higher price, or as a part of a tour package. eg, http://www.alcatraztickets.com (although they have nothing available for the next week). You could also look at a bus tour that includes Alcatraz.

If you're looking more than 3 days in advance keep an eye on the official site as they can have cancellations. Within 3 days it's still possible, but they don't give a refund if you cancel within 3 days I wouldn't be surprised if most people don't bother canceling...

There's only one company allowed runs boats to the island, so there's no point looking at the other companies. Some do run "Alcatraz" trips, but they all involve simply sailing past the island and not actually landing there.

They also used to release a small number of same-day tickets some mornings, although I'm not sure if that still happens.