Only two types of bicycles I know of come with coaster brakes, typically: children’s bikes and cruisers. Both of these are considered ‘cheap’ bicycles. Where I live, coaster brake beach cruisers are extremely popular, but you can pick a new one up for 100 bucks or often much less, brand new. There’s simply no profitable market for any company to improve upon a design that ain’t broken, as they say. You could reach out to any number of custom frame builders to see if they could design something for you, but their time and talent won’t come cheap. Chapman cycles immediately comes to mind:
Chapman Cycles ? Custom frames built in Rhode Island this guy is a creative genius, and I’m sure could come up with something. But you’re not going to likely find anything already designed because, well, you’re looking for a solution without a problem, basically.