I can't see why they're not ready to hang.
Because it's a niche market?
Not sure why you would think that. Do you mean prefinished doors (I imagine the majority of people would rather have factory finished coatings), or the fact that they are 1930s when so many properties are from this era and Victorian or "modern" DIY chain store doors are inappropriate.
Off the shelf pre-finished doors are generally "contemporary" in style so that they appeal to the widest range of potential customers. Take a look at sites like Magnets, Carrs or Howdens and you'll see what I mean. They are also designed to fit current standard openings, which are either metric or the old EJMA (English Joinery Manufacturers Association) Imperial sizes, which really only came into force after WWII. Door openings in 1930s houses may or may not be standard sizes - so you are already dealing with a relatively small market sector IMHO (nuch smaller than Victorian 4-panel doors).
My own experience (as a joiner) is that most people with 1930s style houses (and by that I don't mean Art Deco) seem to want to fit more up to date doors, or to refurbish the originals. I used to have a source for "new" old stock Canadian 3 + 1 panel doors (with 3 narrow panels at the bottom and a high waist) a few years back, but they came unpainted or at best primed. Personally I wouldn't want them clear lacquered because the original timbers were nothing special and the plywood panels (rotary cut) look ghastly to my eye, but there you have it.
If you can find unfinished doors of a style you like whay not get them finished by a professional decorator or sprayed by a cabinet shop before installing them?
From a joinery perspective there are several things against pre-finished doors in an existing opening. Firstly there is a need to chop out for ironmongery such as hinges which can mar surface coatings, especially if you are using power tools of any description. Then there's the fact that doors often have to be shot-in, i.e sized to fit the opening, especially on pre-WWII houses, which may require shortening of the door or even reduction in width - either way you're going to have to refinish the leading edges and/or tops/bottoms
Don't get me wrong. i applaud your attempt to install appropriate (for the house) doors. It's just that finding the doors is probably going to be difficult - finding them pre-finished will probably be a lot more so.
Edit: Just had a look through some of my trade stuff and found that there's a supplier I'd overlooked,
Todd Doors who do 1930s doors of the type we're discussing. Problem is that they're unfinished - that and (fro me) they're a few hundred miles away. Ah, well[/url]