Does a current source practically exist
An ideal current source is a concept, and does not exist, cannot exist. Other useful concepts like an ideal resistor, an ideal voltage source, a 'light inextensible string' or 'frictionless surface' (I loved those two in physics lessons) don't and cannot exist either.

However, not only are these concepts useful mathematically, but they are also very useful as a first order approximation to the behaviour of many real physical things, and can be approximated to a greater or lesser degree of completeness by real physical things.

So we can approximate a current source by a transistor, with the base held at some potential which keeps a voltage across, and therefore a current through, an emitter resistor, which current appears at the collector. This current source has a non-infinite output impedance, varies with temperature, and cannot deliver infinite voltage into an open circuit, but is still useful, in certain applications.

An inductor, with a current flowing through it, at the time the switch controlling current to it opens, is best thought of as a current source. How to cope with the continuous current still coming from it, with the energy behind it to raise the output voltage to any high voltage sufficient to cause breakdown, is a good mental model for how to design the current paths and control components that are there to limit the voltage rise.

When a switch mode converter is controlled to vary its output, 'constant voltage' and 'constant current' are the two main models for what the control inside tries to do at the output terminals.

In the same way, real voltage sources cannot deliver infinite current, real resistors have some residual series inductance, and the only use for a light inextensible string is for tethering a spherical cow.