I get my harmonicas from Anthony Dannecker. He offers Hohner Harmonicas in a variety of tunings. A bit more expensive than a "straight out the box" Hohner, but well worth it as he also fettles them as well so they play better. I have also found his service excellent. I can recommend him. I get special 20's as they are available in a wide variety of keys. For keys less common in folk music, I have stuck with standard Richter Tuning.

You could modify a standard Harmonica yourself. Steve Shaw does, I believe.

The availability of Paddy Richter tuned is what encouraged me to pick up the Harmonica seriously again after 30 years. I have had a Hohner Chromonica in G for some years and I found it very good, though the reeds are going now. However, I really prefer the simplicity of the 10 hole diatonic. Although I can bend notes, I never felt I was able to control them sufficiently to provide those missing notes so the Paddy Richter tuning meets my needs.

Also I have found the Paddy Richter tuning opens up more options when playing modal tunes, particularly for chords.