I use local Facebook groups, charity shops and eBay local pickups mainly. Charity shops around where I live are drying up a bit and/or are wise up to retro values but every so often I get something great which makes it worthwhile to check.
Even up to 3 years ago, you could get deals on Craigslist and eBay, but now pretty much everyone is savvy. Even if someone doesn't want to go through the eBay hassle, there are these eBay proxies now. You drop off your product for them to sell, and if they do, they take an additional cut off what eBay does. Most people I've spoken with prefer these eBay intermediaries over Craigslist, and you still get more money that you would on Craigslist. Flea markets in my area are expensive, too. Pretty much garage sales is all that's left. Over time, you can develop some secrets of your own. For example, I would come up with misspellings and search for those on eBay/Craigslist. I would search for "Sega Saturm" or "Gamevoy". Every now and then I'd get a crazy deal. Maybe others will share their tricks. However, if they're still actively collecting, can't blame them for not sharing.
fatfingers.com is pretty good for misspellings. Also some people would put DC related stuff as "Dream Cast" so that too.