That attachment does not show nickel silver. It lists specialty steels.
If you want nickel silver, just contact a knifemakers supply house.
Make sure to get the matching NS rod from the same supplier so it will match in color.
You can use 4140, or 1018 for guards, but using them for bolster material presents the problem of getting pin stock that will match, or even finding pin stock in that material at all.
416 stainless steel is very good for guards and bolsters. Again, get the matching pin stock from the same knifemakers supply house.