PPSSPP is the most popular emulator for PSP, currently this emulator is available on all platforms, even iOS. For your convenience in finding and downloading roms, below are the roms formats for psp that you can find and download safely.

• ISO: This is the most common format for PSP game files. An ISO file is essentially a disc image containing all the game disc’s data.
• CHD: Already supported on many PSP emulators, compact and simple file compression, easy to use
• CSO: This is a compressed version of an ISO file. It helps reduce the file size, making it more efficient for storage and sharing. However, decompressing the CSO file during gameplay might result in slightly longer load times.
• PBP: This format is used for official PSP game files downloaded from the PlayStation Network. It’s a compressed format containing multiple files, such as the game executable, data, and media.
• DAX: Similar to CSO, DAX is another compressed format for PSP game files. It was used in earlier versions of PSP custom firmware and is less common nowadays.
• ELF: This format contains the executable code of a PSP game. It’s usually associated with homebrew applications and games.
• ZIP/RAR: PPSSPP can also handle zipped or archived formats like ZIP and RAR. These formats might contain the necessary game files, and PPSSPP can extract and run games from them.