There is a taxi rank in front of the main entrance of train station (city side), but I am not sure whether there will be any taxis available all the time. The train station is just within the 17th century extension of the city to the north. Look at the map and you can see the pointy outlines of the defences. So when you arrive, you actually already are in the city.
Haarlem is a relatively small place with most of the sights in walkable distance. I would not take a taxi to get to the Grote Markt which is a leisurely 15 min walk. You can go through the Kruisstraat (more shops) or via the Jansstraat (more quiet).