I prefer potentially serendipitous maxi-cards, eg, when the card was being sent person-to-person with a message, and the card subject & stamp subject & postmark intersect.
When someone buys a postcard of some tourist spot and posts it nearby, you've already got two-out-of-three.
... And you know that the stamp honoring that subject actually was "there" at some point ...
All USPS FDCs are now serviced in Kansas City.
Without independent information, there is no reason to think that any of today's post-issued maxi-cards have been anywhere but a print shop.
... I wonder if there are dealers who still produce maximum cards like these? ...
There are collectors doing their own.
Perhaps, as with FDCs, there are a few artists & small-scale dealers doing something unique on a subscription basis.
But the same tastes & motivations that have led the MNHOG fashion to dominate the market for individual stamps - and that have led the pristine-unaddressed-cacheted fashion to dominate the market for FDCs - would presumably have led the govt-issued 'mint' never-seen-the-mails maxi-cards to dominate that market, too.
The new enthusiasm for artisanal (which I prefer to pronounce 'artist anal') products - I recently tried an artist anal ketchup, and flew back to Heinz - will have a tough time clawing a foothold in maxi-cards, in part because the market is so small to begin with.
Make your own!
Perhaps the way forward is for stamp clubs that are already organizing event covers as a fundraiser to also try a maxi-card, and see who bites.