Local door shop. They pre-hang and bore them for the lockset.
Box store prehungs often have have large and uneven gaps between the door and jamb along with using cheap thresholds. They are usually quite a bit less expensive if in stock but not when you custom order.
therma-tru's website has a dealer locator.
Are you looking for stained or painted?
I'd like to expand on this one a bit...
We have a 9-lite prarie style (sometimes sold as a craftsman style) front door that needs to be replaced. It has two sidelites. Is it generally better to go prehung for something that is 70+ years old, and get inside to look around at the surrounding structure, or is it better to go low-risk and get a carpenter to replace form, fit and function?
I should really post some pics... my fear is keeping the same trim and surrounding areas, avoiding damage and keeping the original look.
I would go with a pre-hung unit with sidelights. Weather stripping etc is way better these days. Get as close to the original size as posssible or go bigger, not smaller because it will be easier to preseve the interior and exterior wall finish. In the 32 yrs i've been in my house. I have replaced every door and window at least once. All my exterior doors are metal and 3' wide now.