i would just ask the staff/owner of your favorite mexican food place(s). many cater but do not advertise it. while i am not that fond of the restaurant - i've had quite good experiences with the barragan's catering (which people like for work events and I don't have a choice). they are located downtown la but provide catering city-wide. i know don antonio's on the westside does not cater.
How about a Taco Truck, complete with all the fixins and homemade Horchata & Jamaica. Give them a call and see what they can do. Great people, family owned and operated, just used them for my 30 year high school reunion picnic.
Las Costas de Guerrero
626-665-6009 or 626-665-6008