Finding a 3DS BIOS (or firmware) file can be a bit of a headache because of the legal "gray area" surrounding emulation. If you are looking to set up an emulator like Citra, What is the 3DS BIOS/Firmware?
The only 100% legal way to obtain these files is to dump them from your own Nintendo 3DS console . To do this, you must have custom firmware (CFW) like Luma3DS installed on your handheld. Nintendo 3ds Bios File Download
Downloaded files are often for the wrong region or version, leading to more headaches during setup. The Right Way: Dumping Your Own Finding a 3DS BIOS (or firmware) file can
You’ll find plenty of sites offering "3DS BIOS packs," but proceed with extreme caution. Copyrighted Material: To do this, you must have custom firmware
: Files used for system-level functions like the shared font, camera support, and the Mii data folder. How to Legally Obtain 3DS System Files
The safest way to obtain a BIOS file for your 3DS is directly from Nintendo or through an official update. You can also try searching for the file on official Nintendo websites or forums, but be cautious of third-party websites that claim to offer BIOS file downloads, as they may bundle malware or viruses with the file.