There's no secret society, but the 3 people you mentioned have very long tenure in the scene, so they've cultivated relationships with other bonsai people and are more aware of the supply chain (for yamadori, for pottery, for wire, for akadama).
Even living in what should be renamed from "Willamette Valley" to "Bonsai Valley", when I started out, I didn't muster the courage to attend local bonsai meetups, so it took me much longer to become aware of the bonsai supply chain. As a result I started with mediocre nursery stock and crappy (also wickedly expensive) options for soil suppliers.
Meanwhile, not only was there better bonsai material and supplies right under my nose that whole time, but also, there was lots of free material / supplies / tools floating around as well. So I am with
@JudyB on this, connecting with other bonsai people in your region (if not necessarily your immediate neighborhood) will get you much closer to straightforward answers to all needs, whether it's a nice JBP trunk or where to find akadama or pumice.
On the other hand, some types of material or supplies are less regional, and yamadori and pre-bonsai can be like this. If your goal is to start with high value material which is also completely raw, like what Ryan, Bjorn an Sergio often work with, then you must muster the courage to reach out to collectors such as the Backcountry Boys, Randy Knight, etc. They're good people with lots of material on hand (Randy has quite a few trees at any given moment). If your goal is to start with professionally grown pre-bonsai (pre-engineered roots, some time given to trunk growth, etc), there are professional growers on this forum (
@JEads for example ) and on instagram. Find one and you quickly find the rest.