List of DB Workshops?

To my knowledge, no one has curated a comprehensive list. I've scoured the internet a few times over the past 10 years so here's what I know.
Please do a search on TB and Google and you'll find this topic addressed several times.
For novice adults, that's a pretty narrow niche because most jazz camps are targeted towards teenagers.

The two camps that I've experienced is Jamey Aebersold's camp. and Lynn Seaton's Double Bass Jazz Workshop at UNT. Lynn's is done for the year, but you can probably still register for Jamey's camp for one or both of the first two weeks in July in Kentucky.
Jamey does a great job in accommodating players of every ability, including beginners and just being in proximity to such talented and developed players is both inspiring and instructional.
That said, there are other options. There's a camp near SF in La Honda that people have spoken about highly on TB over the years. There's also the Stanford Jazz Workshop but I'm not sure how many adults attend.
There's the camp in Port Townsend, WA led by John Clayton which, again, people seem to be very happy with. My sense is that you wouldn't be the only student over 30 but you'd probably be a minority.
There's also a workshop in NYC, I think it's called something like the New York Jazz Academy.
There are others but these are the ones I've given serious consideration to over the years. You can also look for camps abroad. For instance, John Goldsby teaches at a camp in Ireland and a friend of mine attended a camp in Italy the past couple of years.
Good luck!

 

Two that I know of in Southern California is the San Diego Bass Bash and the Rabbath Summer Institute in LA.

 

Summer Camps - Jason Heath's Double Bass Blog

 

The Hammond Ashley workshop in Seattle could be appropriate for you.

 

That was nearly 20 years ago, but it looks like the faculty hasn’t changed so much. It was always a little bit of everything.