I refuse to watch the last Sequel, so I don't know about that, but in the old EU there were several books where Sith cultists are mentioned. In the Darth Bane books they mention that the Sith have a large network of spies, legit businesses, shell companies, etc. to get them intel, technology (ships, vehicles, weapons, etc.), and keep a steady cash flow. Other Sith worshipers collect relics, texts, and even lightsabers that the Sith are happy to relieve them of.
Most of the my favorite SW novels, comics and video games are from the EU. It does have a large list of flaws though and can be hilariously bad. Dark side material, groups, concepts and especially the aspects you mentioned were often imaginative and well done in it. The Darth Bane trilogy was entertaining as was the Darth Plagueis novel, surprisingly.