I use Tmobile as my phone on a prepaid basis. One of their plans has no ongoing cost, you just pay for the minutes you use the phone. I would check their coverage maps to be sure they cover you if you are going a bit off the beaten path up here.

Phones usually are available from them for $20 or so and the card another $10. You will have to add minutes to the card they usually come with 10 minutes or so.

If you are visiting somebody in San Francisco they can pick it up for you, if not it will take like fifteen minutes at the store for them to set you up and send you on your way.