You can often find these files on archival sites like the Internet Archive or community-driven ROM repositories.
While the original 3DS hardware is region-locked, "Region Free" CIA files have been patched to work on any 3DS console regardless of its original region (USA, EUR, JPN), provided the console is running custom firmware (CFW). Where to Find it Shinobi ROM & 3DS (USA) CIA (Region Free)
This is a standard ROM format typically used with flashcards (like Gateway) or PC-based emulators like Citra. You can often find these files on archival