You don’t really want a static variable here, a variable accessible without an instance. You want a GameObject referencing a prefab, and being able to access it from everywhere.
So, create a script with a public variable GameObject, attach to an empty GameObject in your scene and affect the prefab to the variable. Change the tag of the object to something fancy.
Step 2, add a static function to that script which return a gameObject :
public static GameObject GetPrefab(){
return (GameObject.FindGameObjectWithTag("MyFancyTag").GetComponent<MyScript>() as MyScript).prefab;
}