The majority of accomodations in Napa are B&B's and Inns. They are locally owned and individual phone numbers can be hard to find.
There is a wondeful B&B association that you can call for help. The phone links to a local B&B owner who knows which places have availability. They'll give you phone numbers and even descriptions of the places.
Bed & Breakfast Inns of the Napa Valley
Other helpful numbers:
Napa Valley Reservations Unlimited
Napa Valley Tourist Bureau
Wine Country Vacation Rentals