Try your local auto parts chain store section for "repair" parts for things like license plates screws and plugs. Even some hardware store may have those plugs. Just use some stainless flat head screws with them...
I believe the "plugs" you need are actually called wellnuts. And if you have a fastener store nearby, they will carry them. Some have found them at the home centers although HD near me did not. Nor did Lowe's. I got mine at an industrial fastener store. Plus they had every conceivable size of stainless steel screw imaginable too. Good luck!