Classified ads (1×1) have worked best for me; especially the weekleys here in the USA.

Years ago eBay used to have free ads; it was something like "what do you want to see listed" and you would post requests with pictures, but the whole gig bounced on them. They ran it so onlookers would list the items. I also took advantage of all those free online want adds that afe still floating around out there today..

I also utilized the Gypsy pickers in Europe... and hit chip manufacturing/distributing towns heavy in the US with whatever it took to get their attention to see my interests... and wrote every antique/junk shop in Paris, Nice and...

I also wrote articles in UK collector magazines and let as many casino dealers on ships around the world know what I pay for... as well as dealer schools the world over.

I also had a "I BUY CASINO CHIPS" sign pinnsd to me at all antique shows and flea markets and included my business cards in every piece of mail I posted; utility bills and such...

I was also selected for "Trash or Treasure" and that still works for me some 20-years later