I don't have any suggestions beyond
- chatting up the relevant forum,
- joining or starting up a group buy,
- joining or starting a swap, and
- using the "Have and Want lists" in the plant database.
I guess, if you want to sell, you could place ads in the "Classified" forum. I think that "making customers come to you" is a classic problem for any seller, but not really a "problem" that this website is primarily dedicated to solving.
If there really is an untapped group of people here already eager to swap Holiday Cactus cuttings or plants, but who can't find each other, you could try starting a "plant swap" using Dave's "seed swap tools". The "seed swap forum" is for that, but you should also make a thread in the Cactus forum, to publicize it, where interested people would be most likely to see it.
(Before starting up a plant swap, I would FIRST ask around to find how many people might join such a plant swap. They would have to be comfortable shipping cuttings or plants around the country.)
