celebration in napa
The wineries that offer dinner in their caves work with the local caterers - your best bet is to contact one of them (Melissa Teaf, Elaine Bell, Knickerbockers) to learn who might be available to accommodate your party.
This is especially critical as these caves book up months in advance...
I think you will have a much easier time finding a location through a caterer. There are hundreds of caves in the valley and only a few are amenable to renting out to events.
The local caterers know where these caves are and - honestly - the three caterers I mentioned are the tops in Napa so one is just as good as another (I used Knickerbockers for my company Christmas party in the downtown Vinoce winery), but have dealt with all three in the past). They will know the layouts of the properties and which ones are more amenable to your size party.
Also, you might have a party of 12 and find a location that is better suited to a party of 50. The caterer would have suggestions where your party would be more comfortable for its size AND budget.
If you want to see what types of venues might be available, I would suggest searching through HereComesThe Guide.com, although I know for a fact that a lot of wineries do not put their caves on that site because they want to avoid weddings and just stick to catered private events.
My favorite by far is Feast catering out of Santa Rosa.
Click on Affiliates in the upper right hand corner to see the list of wineries by region.
I love Peter McCaffrey's work with Wine Valley Catering in Napa
Click Venues on the website to see the list of wineries.
Both companies work in wineries in both Napa Valley and Sonoma County/Healdsburg.
Slightly different opinion from Carrie, but we agree on Knickerbockers. Good food,
wonderful people, though the food is less elegant. I'm not wild about the food from Melissa Teaff, and I think Elaine Bell's approach is expensive and her concepts somewhat dated.
But that's just my two cents.