Terra Mia Pizzeria just opened up in Tustin (edinger/red hill) and has pizza by the slice. The Laguna hillls locations seemed highly rated on Yelp.
Tried this place out last week, pretty good... the Sicilian thick slice was the best.
Inspired this new thread:
Pizza by the slice
https://www.talkirvine.com/index.php/topic,18376.0.html