Agree with eStarland as a good choice. Outside of a few eBay purchases, been doing all my retro game shopping from them.
eBay can be decent at times for deals and have snagged a few good games for under $15. People using auto bid scripts and bots does get annoying really fast though.
https://www.gameovervideogames.com/store
Did a quick google search and that site seems to have a large collection and decent prices although never done any purchases with them.