Search for terms like "Batocera Full BIOS Pack" or "Batocera V40 BIOS" (replacing the number with your current version). This is the most common host for complete, community-verified sets.
Since distributing BIOS files is legally sensitive, links often change. Use these reputable methods to find a working pack:
Subreddits like r/batocera or r/Roms often have "megathreads" with direct links to archival sites. Step-by-Step Installation Guide