Physical UMD copies of Street Fighter X Tekken for PSP (released in some regions) may be found on secondhand markets like eBay. Owning the disc legally backs up your right to create a personal backup ISO in some countries (check your local laws).
If you tell me which device you're using, I can help you find: for existing fighting games. Vita3K configuration for the actual handheld version. Information on fan mods for Tekken 6. Which platform are you planning to play on? street fighter x tekken psp iso link
version is the closest you can get to the PSP experience. It even includes exclusive characters like to add crossover characters? Physical UMD copies of Street Fighter X Tekken
If you find a website offering a "Street Fighter X Tekken PSP ISO," it is likely one of two things: Many fans have created "total conversion" mods for Street Fighter Alpha 3 MAX Vita3K configuration for the actual handheld version