I used to get MS-DOS games from fellow collectors, thrift stores, and ebay. The thing is I collected MS-DOS games when most people did not so there wasn't that much competition.
For 486 or earlier hardware I got most of that from ebay when it was cheap, local people dumping stuff, found a complete 486 once at a thrift store, Craigslist, and Freecycle before the gold recycling craze killed that.
A local recycler that went out of business ages ago provided me with a ton (literally) of stuff over the few years I visited them weekly. Were talking complete vintage machines, CPUs I could sort and buy by the pound, same with add-on cards. One time he got a stack of old pre 1GB IDE hard drives the owner let me take home and test and then buy for $1-2 a pop and just return the dead ones. Purchased a stack of 5.25" floppy drives from him for like $20 total. I also got most of my IBM PS/2 and AT clicky keyboards from him for like $2 a pop (plastic was pretty much worthless to a scrapper at the time). I kick myself for the items I passed up but to be honest I ran out of space for it all.
An old guy I used to talk to referbed machines for people and he would always get some old junk in as a trade he could not use and dumped them on freecycle. I used to help him fix stuff once and a while and he would just give me stuff he could not sell like compact macs, hard drives that were too small for his use, misc old software and hardware add-ons etc. Heck once I purchased an Apple IIgs from him with keyboard, mouse, drives, and some original software for $20 and found it had a Transwarp IIgs in it plus a 6MB RAM card. He had a box of boxed C64 games but sadly most had mold but I did get GEOS and some other ones.
Freecycle in general scored me my C128 + 1571's and quite a few C64 setups, old 68k mac gear, a few IIgs systems, Atari 800xl and 65xe, misc game consoles (Atari 5200, some 2600 games), some old laptops. I think one guy giving away tractor feed printers (business address) I visited had a foot high stack of shrinkwrapped OEM W95/98 CDs and books heading to the trash before I snagged them. It is kind of shocking what treasure you will get handed when you show up to pick one item up and the people seem happy they are unloading their junk on you.
Craigslist is good for the occasional AT case NIB somebody grabbed from work and wants $20 for. Once I got a box of 20x 50 pin SCSI 2GB drives for $20 which really thrilled me since they were sealed 0 hour drives. Most Craigslist stuff tends to be newer and these days over priced.
Everybody has their own way of finding items, but I find it helps to get contacts who recycle or otherwise happen to get items you might want in their hands. Also keep in mind that the stuff going around these days is not the same stuff I was finding 10 years ago. With the recycling craze most of those old 486 era machines are either in collectors hands or made into razor blades by now. You can always find another local collector to trade your junk for their junk if money is an issue. If money is not an issue you can still find pretty much anything you need on ebay or over at the appropriate computer forum.
Collector of old computers, hardware, and software