We just happen to walk by and went inside. It's across from the Oxbow Public Market just through another set of doors. It's a big room where you can buy wines, taste wines, get a bite to eat, and buy some cheese all in the same area. They are owned by the same people at the SF Ferry Building Wine Merchant.
We just walked by the wines, but we stopped at the cheese area. We got to try 3 different kinds of cheese before deciding to buy one: Tumalo Tomme a goat cheese .53 lb for $15.24. Yikes, it's pricey.
They sell St. Benoit yogurt for $5.25 ($1.5 deposit). Vermont Butter. Straus milk.
They wrap it up for you then you go to the cashier station near the door to pay. Credit cards taken.
The Oxbow Cheese Merchant
610 First Street, Napa, CA 94559