Honest question: how did you acquire this sort of stuff prior to the Internet community?
Official Releases, Third Party Releases (diminishing after the early 80s), and above all magazines like Dragon, White Dwarf, Dungeoneer, Judges Guild Journal, Pegasus, etc. By the mid 80s we had Dungeon magazine.
If you were involved in the right area at the right time with the right people, you could get stuff through fanzines and handouts. For example Lee Gold's Alarums and Excursions. Which I didn't know existed until the 90s and the early internet.
Depending on what you found and had the time for, early computers got a serious workout for random generation of stuff. I still have a printout of a 1,000+ village made with a program found in Dragon Magazine (I think #42). There was some AD&D 1st edition disk for DOS that one could buy.
My usual procedure until the Internet grew big enough was to use the yellow pages to find all book stores, model shops, and comic stores in a town. Go to them and see what magazine and product was available. Then use that to send for catalogs. That how I found Tim Kask's magazine Adventure Gaming in a store in Pittsburgh.