Most of my collection is stuff I found in the wild. Unfortunately there are only stores that sell retro games in my area, one is great and I visit every other week or so, the other sucks (The sell beat up games at full pricecharting price). I'll stop by there once every couple months just too check for games on my wishlist.
Like I said, most of my collection is "wild" finds. I have made a fair number of purchases on Amazon over the years, but that's it for online. No FB marketplace (I have looked there, but usually overpriced junk), mercari, vinted, and my biggest shocker...not a single ebay purchase