Happy to provide any insight I can, although I'm not super knowledgable about white label health supplements. However, if you have a very niche market, then there must be a few key figures and influencers in it. Whether they are companies, or experts, they would probably be the best to reach out to or study.
I think it depends on your product, but I do think finding influencers in your field is the best place to start. Otherwise, maybe try to check out forums or Facebook groups. Try to hunt down anywhere communities of your niche exist!
Or, use Facebook's lookalike audience tool to use an algorithm to find them for you!