If you mean the source code for the emulator ePSXe, it hasn't been released. If you are referring to the actual plugins themselves, the ePSXe authors do not modify the plugins except for the internal ones that come w/the emulator. They just write the main emulator core and other authors write the GPU, SPU, etc., plugins that work with, but are not modified by, ePSXe.
sincerely,
sx/amiga