these bookcases and window seat are custom designed and built in to fit the room. You could either go to a designer and have them draw up something for you or to a Home Depot or similar and have their kitchen cabinet people help you order something to fit.
Good luck to you and thanks for the question
Bookcases, window seat under circle window
adjacent bookcase match up with those of the drawers in the bedroom, implying that this bench was no mistake.