Hi! Just found this forum recently and have finished most of the books now.
I've wondered about where the Animorphs live for a while now.
I checked what you all wrote here and remembered that in book 51 they said the Capitol was over 200 miles away. I'm guessing it wouldn't be too much over 200 miles.
I googled a little, as I don't know much about California (I'm from Germany). From the distance it could be either Eureka (about 209 miles from Sacramento) or San Luis Obispo (about 230 miles from Sacramento), considering that the city is near the coast.
I tried to google deserts as well. The Mojave Desert seems to be not too far from San Luis Obispo.
So, San Luis Obispo is my best guess.
What do you think? Could that be right?