– The official PG Music forums and r/BandInABox on Reddit sometimes share user‑created song files (just chord progressions, not full lead sheets) for standards. These are usually permissible because chord progressions alone aren’t copyrighted.

If you are a gigging musician or a serious student, this collection is an essential time-saver

The most reliable place to find active links or alternate smaller "Real Book" sets that are often better curated. Jazz Guitar Online

Unlocking the Ultimate Repertoire: Band-in-a-Box and Real Book Files

If you already own Band-in-a-Box, you can import your own BIAB song files from legal sources or create them from legitimate sheet music. For the exact 13,000-tune set you mentioned, I’d need more context—if it’s a specific user-created archive, it might be shared in user forums (like the PG Music forum) for private, non-infringing use, but even then, linking to copyrighted content isn’t allowed.