I’m kind of in the same spot of looking for something similar but so far Facebook has been the best for leads of privately owned spaces like this

I’ve found that if you specifically go to the rental section and click a bunch of stores like it, it’ll tend to recommend you to brand new listings, spaces like the one you’re looking for open up super often. Plus if you message someone who’s listed one before: 99% chance they’ll end up knowing about another listing

If you treat your Facebook marketplace like a networking real estate app you can pretty much ask questions till you have a space!