This is considered the "Gold Standard" for two reasons:

Remember: True exclusivity in the emulation world isn't about having a rare file. It is about the skill to extract it from your own hardware. Be a preservationist, not a pirate. Dump your BIOS today, and finally play Pokémon Black 2 in full DSi-enhanced glory.

Downloading these files from the internet is technically a copyright violation, so they are rarely hosted on official sites. However, community members often look to these "exclusive" archives: the DS and DSi Bios Files of #MelonDS | #NDS + #DSi Menu