I guess, for starters, if these doors are not 'modern doors', then a Salvage Yard might be the place to go. However, these days, antique doors are 'in'. Many door companies custom make them but you have to seek them out.
I know this sounds silly but have you check at Home Depot? THey deal with many mfgrs. Also, ask them at the Building Materials Desk where you might go. Also, there are Door and Window companies in the YELLOW PAGES (listed under DOORS.) I can certainly give you a list of Door Companies in my YELLOW PAGES but I think you'd do just as well looking in yours.
I do wish you well. My best to ya and hope this helps.
Jay J -Moderator
PS: Maybe you need to say more about the house they're going in ...