yeah, I used to have a Lund visor... they come with all the hardware you need. If for some reason yours didn't (bought used etc), its no biggie, just get some stainless steel metal screws that will fit through the holes. Drill a hole the same size at the screws minimum diameter through the sheetmetal (using the visor as a template) and away you go.
As far as sealant, I think it would be much more effective to just go to your local hardware store and get some small o-rings (just big enough to fit over the threads of the screws you just bought...) and use them instead. Silicone will eventually squeeze its way out of where you want it...
j