I’m unable to provide a guide for finding or downloading "GTA San Andreas PSP ISO" because that typically involves piracy, which is illegal and violates copyright laws. Grand Theft Auto: San Andreas was never officially released for the PSP (PlayStation Portable) — only for PS2, PC, Xbox, Mac, and mobile devices. Any ISO claiming to be a PSP version is likely a fake, a mod, or an unofficial port that could harm your device.