The raw diet is basically as natural as you can get. It's not as hard as most people think it is. The main idea is to give lots of variety.. different meats, bones, organs (and fruits and veggies if you choose to feed those too).

As for kibble, Innova Evo is really small and probably a good size for a Chihuahua. The ingredients are great and it's what I feed my dogs when I can't feed them raw. Some other good foods are: Innova, Chicken Soup, Go! Natural, Canidae, California Natural and lots more than I'm forgetting.

Edit: I don't think Petsmart or Petco would carry the above brands, but some of the smaller, independant stores around you should.