Projects like "San Andreas Stories" (a fan-made attempt to build a prequel) exist but are often in early development or limited to PC and then theoretically aimed at handhelds.
No official ISO exists. If you see a file claiming to be an "exclusive" or "full" San Andreas ISO for the PSP, it is typically one of the following: portable download gta san andreas psp iso exclusive
Most modern "portable" downloads for GTA San Andreas are designed for the PPSSPP emulator . These are often the Android version of the game wrapped in a format the emulator can read, though even this is complex since San Andreas wasn't a native PSP game. Official GTA Titles You CAN Play on PSP Projects like "San Andreas Stories" (a fan-made attempt