As mentioned, you should have no trouble finding shopping in these ports, as the stores are primarily on the main streets.
In Skagway, most everything is on the main drag (Broadway) or visible on a side street as you walk down Broadway. Plus many of the stores are in historic buildings.
In Juneau, it is very similar. If you walk down the main street by the cruise docks (Franklin Street) you will see a number of stores. You can either stay on this street or explore connecting side streets if you see something of interest.
In Ketchikan, there are stores on Front Street (by the ship docks) and then also on Creek Street. There are plentiful signs to Creek Street. It is all very walkable and interesting as well (various totem poles and artifacts along the way).