If you're unable to find a compatible game file, consider purchasing the game on a different platform, such as Steam or the EA Store, to play Need for Speed: Underground 2 on a more modern device.
While the PSP version of Underground 2 (often titled Need for Speed Underground Rivals in some regions or a ported experience) differs slightly from the console version, the core DNA is the same: Need For Speed Underground 2 Ppsspp Zip File