I use Karmel shuttle often: http://www.karmel.com They tend to be a little more expensive than SS and PT but don't make a lot of stops and detours in parts of LA you never knew existed! I'm not sure if they are permitted to take you from LAX-SNA but if not, they can take you from LAX to the Hilton across the street.

The other option not previously listed is the coach from LAX-D'land then DL-SNA: http://www.coachamerica.com/grayline...s_shuttles.cfm Total cost $35 booked online.

BTW, only Metrolink trains stop in Tustin. Amtrak and Metrolink stop in Santa Ana and Irvine but there's no convenient transportation to SNA from there except a cab.