There’s one here in town that’s a small used car dealer but also offers a locator service, if you give them an idea of what you’re looking for and if one of the 3 dealer auctions has it they’ll get it for you.
Dealer group that’s mostly 25mi south of us except for a VW and RV dealer here will also look around for the vehicle you’re having trouble finding. Co-worker bought his girlfriend’s car through them and they did an inspection and fixed the few things wrong with it. There have been services like you describe in the past but the only one not connected to a dealership closed operation at the end of last year.
Credit unions do offer a car buying service for new or used compared to Costco which only deals in new cars.