When we bought our house, we felt fortunate that our house retained many of its original details - hardwood floors (protected by carpeting), original moulding, original kitchen, and no strange reconfigurations to the floor plan. When we were shopping for houses, we saw many houses which had undergone horrific "remodels" in the 1970s where all the molding was removed or horrible kitchen renovations complete with wood-grain formica cabinets and rust-colored countertops. So, although our house needed some cosmetic upgrades, we at least didn't have to undo anything that had been done previously. The majority of our work was painting, floor refinishing, and landscaping but nothing too serious. We also knew from the beginning that if we were going to stay in this house long-term, we would have to add on and do a major renovation. As a result, we only did a minor kitchen renovation (painted walls and cabinets, vinyl black and white tile, and new appliances) to make it livable.
As we are preparing to do change a lot of things about our house, we are making a big effort to keep the character of the house which we love. In our plans we stipulated that all moldings in the house should be matched in the addition including baseboard, window molding, crown molding, and wood floors. We are also looking for ways to make small details like the interior door knobs match our existing original glass knobs and brass back plates. We can probably achieve this by reusing the existing closet door knobs on the main hallway doors for the bedrooms so when you look down the hall, it's not obvious which are the new rooms. Over the years, we have replaced some chipped or broken knobs by purchasing new ones on eBay or at local garage sales. We'll probably do that again if we find we need more to match, or we'll check out some local salvage places.
DC has several great local options for architectural, including Community Forklift, The Brass Knob, The Brass Knob Backdoors (for big stuff), and Habitat for Humanity ReStore. Habitat has ReStores around the country so check their website for local options. You can also Google - "architectural salvage [your city name]" and you'll probably find some good local options. Many places (including the Brass Knob) offer nationwide shipping so if you find what you need in their catalogue (or they can find it for you), you can get it sent to you. Price-wise, you'll probably do better at Community Forklift or the ReStore, or even eBay as opposed to the speciality options, but you may be more likely to find it at the speciality store. Community Forklift and ReStore also have modern items such as appliances, new hardware, and other building supplies. I'll keep you updated on our efforts to make changes in keeping with our 1940s house as we move along with the renovation.