Most of my stuff I bought cheap in the late nineties when it was just old (not retro yet) and nobody wanted it. Now I justify paying today's higher prices for stuff I need by selling stuff I bought cheaply ten years ago.
Then there are bike jumbles and local small ads, but normally when I need something I go to one of my cupboards and rummage...
10+ year bump... I see a lot of hobos & vagrants on nice vintage mtbs, they usually respond favorably to “can I buy that bike”
Another option is college towns right after everyone leaves for summer break.
Edit: ww2 baby boomers are another good source... they’ve hit the age of no biking for health & safety reasons, the bikes usually have spider webs on them
Local adds. Find badly advertised bikes or bikes that are worthless but have interesting parts. Just got a Shimano "last invander" for €50 with another free bike included in the sale. For some reason the budget shimano bike had full xt m730 slr stuff on it which I doubt was there originally, looked like it'd been transfered. The other free bike had wolber titanium wheel rims with stx stuff which I'll probably sell....
Find alot of dumped bikes since I'm in a student town, pretty nice stuff can be found. The local dump, boot sales etc.
here's my list , eBay, Facebook, marketplace, gumtree, cycle recyclers , council dumps, scrap men , scrap metal yards , cycle charities , job lots local auctions , house clearance's , car boots , cycle shop trade ins , free adds, m o d
Try all your mates who were into mtb and then got married and had kids....most will be to tired or under the yoke to ride and the rest will now be too unfit to even lift it out the garage for you. The wife will love you for getting rid of "that junk" he hangs onto and you mate no longer needs to feel guilty every time he goes into the shed for a new slab of Pampers. Win all round.