Constant voltage sources are a lot more commonly seen than constant current sources. In many (maybe almost all) real circuit designs, you want to have some kind of well-defined voltage no matter what load (resistance) you put across it. This kind of thing is provided (to a very good approximation in reality) by a battery or power supply. One sees current sources a lot less often, but they are used as components of some circuit designs (possibly we will see some later).