@ebeyer
In that case, i'd keep an eye on GOG, they often give away free games that use SCUMMVM or DOSBOX, which can be played on retropie. (They're licensed to sell the games they're selling)
Speaking of SCUMMVM and DOSBOX these platforms also have hundreds of freeware titles available.
https://wiki.scummvm.org/index.php/Where_to_get_the_games
Furthermore, you might want to look into some homebrew for the various systems.
And lastly although not free, fanatical.com which is is also a licensed storefront often has the entire Sega collection on sale for roughly 5 bucks.