S&W doesn't sell new inserts, since these are cast into the sight.
You either buy an entire new front sight (on pinned-in sight guns) or you cast a new insert.
Any gunsmith can cast in a new insert in about any color you want.
If you're handy with small jobs, you can even do it yourself using epoxy glue and a tiny bit of model airplane paint.
The fastest fix is to just paint the sight with model paint. Fluorescent Red shows up nicely.