I haven't seen anybody mention Midwest Photo Exchang, in Columbus, Ohio. I have had excellent results in buying used from them. When I bought my first 4x5 (an anniversary Speed Graphic) I called them from their ad in Shutterbug. They were quite helpful to a newbie to 4x5, and helped me get started with a clean 127 Ektar in supermatic shutter, in the box, and with the Kodak quality control paper signed in 1941, for $35.00. I didn't have the money for a good coated lens, or the experience of what to look for. (Now my basic 4x5 lens is a Linhof select Voigtlander 150mm APO Lanthar)
Over the years I have bought a Korona 5x7, Calumet C1 8x10, film, holders, and many Linhof access from Midwest. The Staff is extra helpful, and they knoe their stuff. If they don't have what your looking for, give them a week or two, and some how they find it.
I also give high praise to Jack's, I believe they are out of Muncie, In. (I also found them in Shutterbug a few years back.)