Since there is no official ISO, what you are likely looking for is a Homebrew Port. Independent developers have created fan-made versions of Subway Surfers that run on the PSP.
Purpose: To clarify the availability of Subway Surfers on PSP and suggest safe alternatives. subway surfers psp iso upd download link
Subway Surfers launched in 2012 for iOS and Android devices. By that time: Since there is no official ISO, what you
If you want a similar endless-runner experience on your PSP, consider these legal, playable-on-PSP titles: Purpose: To clarify the availability of Subway Surfers
If you specifically want to use your PSP and are comfortable with custom firmware, search for open-source runner games on GitHub (e.g., “PSP runner homebrew”). These are legal to download and install.
Downloading copyrighted ISOs for games you don’t own is illegal in most countries. Since Subway Surfers is still commercially available (iOS, Android, even some Windows Store versions), distributing or downloading an unauthorized PSP version would violate SYBO’s copyright.
Homebrew (original code) is legal, but distributing ripped game assets isn’t.