While the file is officially a part of Sony's PSP firmware 6.60, it is frequently shared by the community for use in other emulators.
For most emulators (RetroArch, DuckStation, OnionOS), you must place it in the /BIOS folder. Download Psxonpsp660.bin
gingerbeardman/PSX: Various things for PSX (PlayStation 1, PSone) While the file is officially a part of Sony's PSP firmware 6
If you cannot dump it yourself, most users find this file through the following types of sources: Download Psxonpsp660.bin