Since the game is no longer in active production for the PSP, many archives host the file. If you choose this route, use a VPN and an ad-blocker. Avoid any site asking for a credit card. Look for these file details:
Warning: Do not search for "free download no survey" links. These are often malware traps. Stick to well-known archival sites (like Archive.org or CDRomance).
This is the most sensitive part of the article. The "PPSSPP file" technically refers to the ISO file (the disc image of the game). You have two legal avenues to obtain it:
To run any "Storm 2" mod, you first need the original Ultimate Ninja Impact ISO. You must dump this file from a legally owned PSP UMD or purchase a digital copy. Search for "Ultimate Ninja Impact ISO" on archive sites (always check copyright laws in your region).