Umm, how do I put this delicately? It is a niche market. Yeah, that's it. A niche market, that's why no one seems to sell I/O plates by themselves.
The less-polite translation is that because I/O plates come with most cases and motherboards, not a lot of people will ever need more than two per system. Ergo, businesses don't see a need to stock or sell them as a replacement part.
If your motherboard has a standard ATX port layout, your local computer shop will probably be happy to give you one they have laying around. But if your board has a special layout, like most enthusiast boards of the last couple years, your only hope is to e-mail the board maker and suck-up like a mother, or make your own.
BHD