I don't know if there are replacement sights available for them. Since they look like Novak designs, you may want to contact them and see if they have any options.

Or you could do what I did. The tritium on my Beretta 92FS are completely dead, and because of the integral front sight I don't have any relamping options, so I just painted over the vials. Black rear, fluorescent orange front (with a white base coat). You could also just use a black Sharpie over them to see how it works for you before trying paint.