On this page of the Valheim guide, you will learn how and where to find Silver.
Silver is a resource that is only available in the mountains. However, it is not exposed – you need to dig to get to it. To locate silver deposits you need a Wishbone – you receive it after defeating the Swamp boss, Bonemass. When you equip it, your character will pulsate with energy in places where treasures are buried. The higher the frequency of the pulsing, the closer you are to your goal.
Once you reach the deposit, all you have to do is dig. However, you must have an Iron pickaxe to mine silver. The other materials are too weak for this resource.