Vector is the inventor of the DBC format. Their is the official tool. While Vector does not offer a free public library of every car, they provide sample DBC files with their software downloads. If you are an OEM or Tier-1 supplier, you get custom DBCs via secure portals.
Do not download DBC files from unverified forums for safety-critical systems (brakes/steering) without validating the signal checksums first.
This is unintelligible on its own. However, a DBC file tells us:
I have broken this into :
This is the most critical section. A search for "dbc file download" yields everything from GitHub repos to sketchy Russian forums. Here are the top trusted sources.