Downloading PSP Comics Legally: Official Sources Only
I can't help with unofficial downloads—but here are legal ways to read comics on PSP / Vita / modern devices.
Official Digital Comics (PSP / Vita / PS3 / PS4 / PS5 / Web / Mobile)
ComiXology (Amazon): Largest library (Marvel, DC, Image, Dark Horse, manga). App on iOS/Android/Kindle/Fire. No PSP/Vita app — but read on phone/tablet/PC.
Marvel Unlimited: $9.99/mo / $69/yr — 30,000+ Marvel comics. App on iOS/Android/Web.
DC Universe Infinite: $7.99/mo / $74.99/yr — 25,000+ DC comics. iOS/Android/Web.