Sony still holds copyright. You must dump it from your own original SCPH-5500 console (or SCPH-5501/5502 if using appropriate region). No legitimate “free download” exists.
The only officially legal way to obtain this file is to dump it directly from a physical SCPH-5500 console that you own. playstation scph5500 v30 japan bios scph5500bin hot
: It manages the startup process, including the iconic Sony "diamond" logo and sound. Sony still holds copyright