Where do I get those?
I don't know where to find the keys themselves, but I know where to find apps that can sign stuff.
One even works on the PSP itself. You load it up on the PSP, and use it to sign homebrew. Due to technical limitations, it can only sign basic stuff like emulators.
I can only think of one reason someone would make a PSP app that signed PSP apps: a gigantic middle finger to Sony.
Well, maybe another. For the lulz.