Go to Mexican or Taco Trucks in & around Palo Alto
El Grullense on El Camino is pretty authentic and cheap. I also like Taqueria Azteca (located inside a dive bar called Antonio's Nuthouse) on California Ave. The carne asada tacos there are great. I've seen some taco trucks around Xerox PARC (Foothill/Hillview). There's construction right across the street there. I haven't tried anything from the taco trucks though.