Here is the official web site:
and Mark McIntyre suggests: http://www.blueandgoldfleet.com/az.htm#webtix
and Marguerite suggests: http://www.sanfranciscovisitor.com/alcatraz_island.htm
and Michele wrote: I checked with the advanced ticket sales. Right now (June)they are selling out at least 10-12 days in advance and expect that to continue through September.