Professional Headlamp
That might mean that you need to find the headlamp that you like and they buy a separate strap for it. But may be that is already what you intend to do. As far as plain black goes, that might be more of a challenge. When I see "professional headlamp", I try to imagine you uses headlamp in the course of their employment. Some medical professional use things of that nature. However, I would envision those to be white, not black. There are others that use headlamps in a professional setting, but many of those would be attached to helmets rather than using a headstrap. Possibly, if all else fails, get a headlamps and dye the strap black.
Another thing that you may want to do it contact the manufacturer of the headlamp that you are interested in. Ask them where they get their straps from. Then contact the supplier, maybe they can make a plain black one for you.