Voltage source as we know is the battery but what is the current source? Is it just a voltage source converted to a current source on paper to simplify the computation of the circuit or it really exists and if it exists what is its example ?
Neither voltage sources nor current sources exist.
However, they can be described mathematically as a perfect device. This helps to simplify description or analysis of circuits.
Many physical devices are sufficiently close to the behaviour of one or the other to be called that with little error. Often the model of a voltage source with a series impedance, or a current source with a parallel impedance, only holds over some small linear range, about which we want to describe the behaviour.
For instance a low impedance battery, lead acid or LiPo, is very nearly a voltage source, as is a charged capacitor, or the emitter of a transistor with a voltage on the base (an emitter follower).
A high voltage battery with a large resistor after it is very nearly a current source, as is an inductor with current flowing through it, or the collector/drain of a transistor supplied with base current or gate-source voltage.
A voltage source supplies a predefined voltage, likewise, a current source supplies a predefined current. Ideally, the output does not depend on the load, in practice however it does. This is because real voltage sources don't have zero internal impedances and real current sources don't have infinite internal impedances.
If you look at basic definition of "voltage" and "current", you will understand that voltage:- Is the difference in electric potential energy between two points per unit electric charge. current:- Rate of flow of charge (electrons)
So Voltage can be there without the current being there, thats the reason that we always give values to power source by rating the voltage. So when you say that battery is voltage source, its correct but the same battery is current source. But the current will never flow until the circuit is complete, and also the current will vary depending upon the load.
for eg:- with load of 1kohm, 9V battery will be producing 9mA, but with load of 10kohm it will produce current of 0.9mA.
The voltage mostly remains constant and current varies depending on load, hence we usually say its a voltage source (saves hell of complexity), but when we are talking about source which is designed to give constant current we will call it as current source.
So basically when you buy a battery you buy a source of energy, which has the potential(voltage) to make the electrons flow(current) in the circuitry.
So a current source example would be V-I converter, which will make sure that constant current is provided at output even if the load varies.
PS:- I believe your question was more about understanding the concept than technical. Hope i was able to give you some perspective.