A previous topic mentioned getting arepas at a recent festival in San Francisco. I tried one for the first time last summer and was instantly addicted. Unfortunately I haven't been able to find them since then. I contacted them and they said they would have a booth at several events this summer:
SF Carnaval the weekend of the 23rd of May, Castro St Fair, Folsom St Fair, Bay to Breakers (May 17th at the polo fields) and possibly Outsidelands again.
Pica Pica: http://www.picapicakitchen.com/
Pica Pica Maize Kitchen
610 1st St Ste 5, Napa, CA 94559