How to get the player's mouse in a global script?

Although I'm not sure why you want this in a server script, the only way I know how to do it is with RemoteFunctions.

Scripts

Server Script:

local GetMouse = Instance.new("RemoteFunction")
GetMouse.Name = "GetMouse"
GetMouse.Parent = game.ReplicatedStorage

local player = script.Parent.Parent.Parent 
local mouse = GetMouse:InvokeClient(player)

Local Script:

game.ReplicatedStorage.GetMouse.OnClientInvoke()
    return game.Players.LocalPlayer:GetMouse()
end