Individual developers and preservationists sometimes host, or link to, decrypted files on GitHub for educational purposes or for use with open-source emulators.
If you already have encrypted files (like those dumped directly from a physical cartridge), you can decrypt them yourself using specialized tools: where to find decrypted 3ds roms
If you have a modded 3DS, GodMode9 can dump your physical cartridges directly into decrypted .cia or .3ds formats. Individual developers and preservationists sometimes host
ROMs often come compressed in .zip , .rar , or .7z archives. Extract them to find the .3ds file inside, but never run setup wizards inside those folders. How to Decrypt Your Own 3DS ROMs (The Legal Method) or link to