Redump.org is the authority on disc preservation. They do not host ISOs, but they provide verified (CRC32/MD5/SHA-1) for every PSP game ever released.
Advanced users can use ROM management tools like OfflineList with DAT files from Advanscene to batch-verify entire collections, automatically identifying bad or missing files. index of psp games iso verified
Once you have downloaded an ISO, the real work of building your library begins. The modern process is significantly easier than it was in the past. Here is a straightforward workflow: Redump