Data Hoarder

Various websites hosting individual romsets, a good chunk of it is on archive.org, there are projects like mariocube, basically a lot of different places depending on what you're after. And there are some private invite-only data troves the scale of myrient as well.

Of note, the preservation scene uses dat files to distribute metadata about the data in question, allowing managers like ROMVault to take a pile of files and rename/move them into a file structure designating what they are and confirming that they're undamaged. If you're going to start trying to collect, look into doing it this way, rather than trying to just mirror someone else's work.