They usually reside as children at indexes 0 and 1 in the container. There doesn't seem to be any API getters for them.
You can access them directly like this.
var h_bar = get_child(0)
var v_bar = get_child(1)
h_bar.connect("value_changed", self, "h_change")
v_bar.connect("value_changed", self, "v_change")