Na Kampe is a street in Prague. Found in the upmarket area of Mala Strana, it was used by the Outlander team for many of the outdoor scenes set in Paris.
A good way to find this location is from Charles Bridge (Karluv most). Cross over the river to the west bank. As soon as there is dry land under the bridge watch out for stairs on your left. If you walk to these stairs and look down you will see the, immediately apparent, Outlander location. At the bottom of these steps, a quick look around and you are bound to see more of this familiar film set, including the tunnel under the bridge.