The first thing I'd do is find a forum, group, discord, whatever of people who dabble and deal in this kind of thing. Starting with YouTube channels and follow the strings. Said groups almost always have a "deals/trade/buy" subsection somewhere.
But you will be paying for "known good" items.
If you want to get deals you have to troll eBay, local auctions (this is a great source, especially if they're not well advertised/described - look for photos of piles or boxes of "equipment" where you can recognize something like an Apple II sticking out), estate sales (many of the people originally involved are now dying), government sales/auctions (schools and other government places that are shutting down/moving are a great source).
Get to know the people in your area who traffic in used computer gear and let it be know that you're in the market for "really old" stuff. Eventually your network will get large enough that you'll start to hear about things.
Start an electronics disposal side-business. Basically advertise that for $x per pound or machine or whatever, you'll come and clear out electronics. 90% of what you get will just be stuff you haul to the local electronic disposal place (charge enough to break even+) but now and then you'll get a treasure-trove of really old stuff.