Speaking from inexperience (since I don't stock dietary supplements), I'd recommend two things:

(1) Establish your brand presence online first.
(2) Produce a consistent sales track record, converting leads to shipments through your sites.

With a professional looking brand ready to go and convincing sales data to point to, why wouldn't retail stores want to include you?

The approach I'm suggesting would involve a number of components -- from graphics to web design to written content to a brand name and domains for lead generation. Personally I can provide answers for only a few of those areas; and, of course, I have no idea how far along you are already.