Here are some websites worth taking a look
Blues societies and associations
Jorge Puente: mic builder and customiser and leather craftsman for harmonica accessories (Spanish)
Lone Wolf Blues Company. Effect pedals designed for harp.
Skip Simmons amp repair. Got a beat-up Masco from EBay you want to fine tune for harp? He's the man.
Small Parts. Not a harp related site, but a good place to get screws for reed replacement and other stuff