Is there a way of finding an instance that is descendant of a service?

You can't do this on game, because it has some RobloxLocked instances. Attempting to index these will error.

You can however run the following function i made, over workspace and other instances. It will return everything inside a single table.

local function GetDescendants(o)

    local AllObjects = {}

    local function FindChildren(Object)
       for _,v in pairs(Object:GetChildren()) do
            table.insert(AllObjects,v)
            FindChildren(v)
        end
    end

    if o then 
        if typeof(o) == "Instance" then
            FindChildren(o)
        else
            warn("Object is not an Instance.")
        end
    else
        warn("No object given.")
    end
    if #AllObjects == 0 then
        return nil
    end
    return AllObjects
end


local AllTheInstances = GetDescendants(workspace)