Are you in any national facebook collecting buy/sell groups? Outside of ebay, I often find buying from other collectors and resellers is the way to go. Some groups I recommend- Retro Universe, Filthy Games, Collectors of America, Planet Retro, and many more.
Even better- do you have a local/regional retro Fb group? Living in the PNW, we have two major communities- Portland Retro Gamers and Greater Seattle Area Retro Gamers- I've made some great friends and gotten some great deals on games and consoles over the years. It's how I got my Vectrex!