You have to search on Craigslist a lot. Put in searches around your area and then save them and have alerts sent to your email.
Letgo has been good to me. I have found 6 or 7 that way.
Facebook marketplace. Do a search and get alerts to items listed in your area.
Get to know all the arcade collectors within 200 miles. A lot are found and then traded or sold cheap to friends.
Also tell everyone you meet you are looking for them, sometimes you'll find a few.