A basic Google search turned up this page on the Golden Spike National Park website:

Nevada’s silver spike was temporarily returned to Virginia City jewelers Nye and Co., who polished the spike and engraved one side, “To Leland Stanford President of the Central Pacific railroad. To the iron of the East and the gold of the West Nevada adds her link of silver to span the continent and wed the oceans.” The spike was then delivered to Stanford and eventually placed along with the Golden Spike in the Stanford University museum.

There was also an iron spike plated in gold and silver sent by the Arizona Territory:

It is unknown what happened to Arizona Territory’s spike immediately following the Ceremony. Today, it is owned by the Museum of the City of New York.

So depending on if you mean pure silver, the spike is in Palo Alto. The partly silver spike is in New York. There is one Golden Spike that is missing:

The whereabouts of the second gold spike is unknown. It has been speculated that the spike was given to one of the Union Pacific dignitaries, but there is no mention of the spike in any memoirs. It is also possible that the spike was returned to the News Letter. If so, the fate may well have been the same as the newspaper company, when, in 1906, the San Francisco earthquake and subsequent fire destroyed the News Letter building.