Honestly your best bet is to use the US Government's retrogaming archival project and pull roms from that. Might want to get in touch with them to see if they'll permit you to include their collections in your GitHub but it should be ok if done for archival/preservation purposes.
Hi there, you meant archive.org..LOL thank you.. I knew that page...
The Internet Archive is not run by the US Government. It's run by a private non-profit organization, and paid for by donations.
Also, it's not actually 100% legal. It's sharing a lot of software and media that's still covered by copyright, but nobody cares enough to do anything about it (generally called "abandonware"). For example, I just found "Burnout 3 - Takedown" for the original XBox, a game from 2004. I can guarantee to you that that game is still covered by copyright.