I think the best way to find these guys is to Google them and find their Twitter or Facebook account, or find them on Yelp. Most of the good ones give a weekly schedule.
In the Irvine area there are a bunch of food truck gatherings for lunch:
Wednesdays at Teller Ave, just south of Michelson and west of Jamboree, has about 8 trucks.
Thursdays at the OC Fair has about the same amount (and they also have a gathering on Wed nights). Here's a link for this one: http://ocfair.com/qr/food_truck_fare...reThisWeek.jpg
Fridays at Armstrong and Barranca, southwest of the District at Tustin, has about 5 trucks.
damnit, lobstatruck and grilled cheese truck are here on wednesday at the same time as the game. blahhhhhhh...