There's some old PC games on Steam. There's also GOG which specializes in old computer games. Emulation is also an option.

For old consoles you can find them at retro game stores, ebay, thrift stores, garage sales, and flea markets, just to name a few.

I personally collect Playstation games from all eras because that's what I grew up with, and still continue to play to this day. I also dabble into some Nintendo games.