What's is current source in practical ?

[Nhan95]

In theory, the current source is a source that supply a constant current for a circuit. But Does it exist in practical ? I only see voltage source such as cell, battery.

[Rerouter]

a solar cell can be though of as a current source, the exchange of light to current is defined,

[Zero999]

Static electricity generators, such as the Van de Graff generator are constant current sources.
http://en.wikipedia.org/wiki/Van_de_Graaff_generator

[LvW]

In theory, the current source is a source that supply a constant current for a circuit. But Does it exist in practical ? I only see voltage source such as cell, battery.

Each current needs a driving source (potential difference, charge separation, unbalanced charges). With other words: No current without voltage.
Therefore, each practical realization of the device called "current source" is a voltage source with a very high internal resistance (static or differential).
Hence, this source enables a current that is primarily determined by this internal source resistance. Hence, this current is (nearly) constant and not dependent on the connected load.   

[notaroketscientist]

Nicely explained

[TimFox]

Any "black box" with two output terminals can be considered a voltage source or a current source.  For any given DC circuit, the output resistance is the same for both descriptions:  in series with the output for a voltage source or in parallel with the output for a current source.  Practical sources are considered voltage sources if the output resistance is less than the load resistance, and current sources if the output resistance is greater than the load.  A very simple practical current source is a JFET with gate connected to source in series with a battery, which will pass the roughly-constant zero-bias current Idss for reasonable output voltage.  The only actual power sources that are natively current sources are the solar cells mentioned above.

[Sigmoid]

Ultimately, if the question is "how do I make a current source", or "how do I drive a load that requires constant current", there are two kinds of "real" current sources in real circuit applications.

The more generic one is a semiconductor device acting as a current regulator (ie. uses negative feedback to keep the output current constant). You can get ready-built current regulator ICs, even two-terminal current sources (called current regulator diodes) with internal feedback.
These will keep the current at the set value as long as the voltage necessary to do this is below the input voltage of the current regulator (actually below a somewhat lower voltage, as the regulator itself has a minimum voltage drop too).

If however you have a static input voltage, and a static load (such as when you are driving an LED from a battery), then you just set the current using grade school circuit design principles, applying Ohm's law and placing a resistor of an appropriate value in series with the load.