As was mentioned above, I think many non-fragheads become aware of them simply as a result of wandering into stores that sell high-end merchandise. Larger cities have department stores like Nordstroms that attract clientele that expect to see expensive perfumes : The wall of stuff that they have at the store in Portland, while not always uber-niche, is pretty impressive, and enough to tickle any curious person's imagination, as well as their pocketbook.
I suspect the really rare stuff is, simply by it's nature, only sold in a limited number of stores, and that their clientele are also made aware by the sales staff of the uncommon nature of the goods on sale.
While many purchasers are probably not aware of all the ins and outs of niche, I suspect they can tell when they like something, and if they are of the sort with plenty of ready cash, out the door they happily go with a bottle of something that you and I might spend weeks researching.
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