It does seem like there is a production diamond frame, new or used, available somewhere to fit 99.9999% of humans, and you can get that last perfection of customization of some set of those frames from how you paint it and/or what components you hang off it to get the bike you're looking for.

Or, you can get the frame personalized to a fraction of a millimeter in all dimensions so that a XXX gruppo-in-a-box plus a saddle and seatpost finish the job and there you go.