BIOS files are copyrighted system software owned by Sony. You should legally only obtain this file by extracting it from your own Sony PSP firmware. If you'd like, I can:
The file PSXONPSP660.bin is a high-performance extracted from PSP firmware 6.60. It is widely considered the "ultimate" BIOS for PS1 emulators like RetroArch , DuckStation , and PCSX ReARMed because it is region-free and includes Sony's official performance optimizations. Draft Post: How to Use PSXONPSP660.bin for PS1 Emulation download psxonpsp660bin top
— You can dump your own PSP’s BIOS legally using homebrew tools (e.g., PSPident, DumpBIOS). This is the only fully legal way to obtain the file. BIOS files are copyrighted system software owned by Sony