It completely depends on the game. Many PSP games just use the system font that's stored inside the PSP in flash0. Some games use their own font and finding it is tricky. In the game Yuusha no Kuse ni Namaikida Or 2 the font was stored as a GIM image and it could easily be converted to a PNG using GimConv. But most games probably don't store their font as a GIM. Most of them probably store it in a binary file somewhere on the UMD/ISO, possibly compressed.