Run a Google search for "tours" or "tourism" and the name of the port. You should come up with a bunch of choices. Booking "off-ship" (on your own) is usually much cheaper. Read as many reviews as possible to get a good handle on what the tour includes and what it doesn't, as well as any extra costs, etc. Also, book for early tour times, as you don't want to be late returning to the ship. If you book off-ship, the cruise line is under no obligation to wait or get you to the ship. Whereas, if you book through the ship, you have a bit more of a guarantee that the tour is reliable, of good quality and the cruise line will either hold the ship, or get you to the next port at their expense ....