Because of the large number of agricultural workers from Mexico and other points south, the Tri-Cities [1] have an abundance of Mexican restaurants. The finest of these is probably Casa Chapala #1 (107 E. Columbia Dr., Kennewick, 509/582-7848) with authentic Mexican food and ultra-fresh tortillas. They also have restaurants in Pasco and Richland.
The big Mexican grocery store is Mitiendita Mexicana (106 10th Ave., Pasco), with the popular Panaderia la Reyna bakery right next door.
An even bigger Mexican bakery is Panaderia Colima (4th Ave.); if you don’t speak Spanish you may be the only one here who doesn’t.
Links:
[1] http://www.moon.com/destinations/washington/columbia-gorge-and-wine-country/tri-cities