The only legal way to obtain this file is to . You can’t just download it from a manufacturer's site because it contains copyrighted Nintendo code.
Request: Add DSi BIOS + Firmware + NAND for MelonDS ... - GitHub
This file is a binary dump of the system software that resides on the Nintendo DSi hardware. While the DS-mode firmware is relatively universal, the DSi firmware contains console-unique data and configuration. It is the "brain" the emulator needs to load the DSi Menu, manage DSiWare, and handle the improved hardware specs of the DSi (like the cameras and faster CPU). Why do you need it?