I took a screw out of my PA-300 to find that it's a #8 machine screw, 32 threads per inch, measuring 11/32" in length under a flanged 1/4" external hex head.
There is what you may find to be a similar-enough screw available at McMaster-Carr:
Please note that this screw is 1/32" longer than the screw that I removed from my PA300, which amounts to one additional thread and thus one additional turn to install or remove, and that this screw has a serrated flange whereas the screw I removed from my PA300 has a plain flange.