No such license is required. I have bought grapes in both Paso Robles and Carmel Valley from commercial growers without any issues. I do find that growers are generally more receptive when you shop for larger quantities - e.g. a macro bin (which is 1000 pounds plus or minus).
That's true in the east as well. When I lived in Upstate NY, I purchased from 2 Finger Lakes vineyards that would sell in small quantities, and I knew 2 winery owners who would sell excess juice in 50 gallon barrels.
After moving to NC in the 90's I found a couple of small growers in western NC, but didn't like the quality of the grapes, so I used each only once.
Since 2019 I've been part of a group buy that purchases a collective multi-ton collection of grapes from Lodi (through Musto and Gino Pinto), and last year
@VinesnBines sold me 6+ lugs of her southwestern VA grapes.
Finding home winemaker-sized batches of grapes for sale can be tough. I count my good fortune.
The Durham NC-based group I belong to buys 3+ tons of grapes each fall, which gets us a better price on shipping. Ya'll on the west coast should count YOUR fortune that you're not paying for shipping across the country!
We have folks driving 3+ hours to get to Durham to pick up their grapes. It's worth looking for local winemakers to get macro-bin loads.