The non-profit digital library Internet Archive (archive.org) is the most prominent source. Due to its legal status and dedication to digital preservation, it hosts a vast collection of PS1 ROMs in various formats. A search for "PS1 PBP" on the site yields numerous user-uploaded collections. For example, one can find a complete collection of Crash Bandicoot PS1 games pre-converted to both PBP and IMG formats. There are also links within these collections to larger archives of hundreds of PS1 games.
Safe archives offer direct HTTP downloads or torrent links. Avoid any site that forces you to download a proprietary "installer" or tool.
Creating or curating a PlayStation 1 (PS1) ROM archive, especially one focused on PlayStation Portable (PSP) Playable (PS1) PBP (PlayStation Portable Package) files, involves several considerations. Here are some helpful texts and guidelines you might find useful:
A highly efficient open-source compression format used heavily by RetroArch and MAME.
A PBP file is divided into several indexed sections: