What are some practical examples of a constant current source?

Photovoltaic cells and betavoltaic cells are examples of devices that produce more-or-less constant current up to some maximum open-circuit voltage, just as a battery produces more-or-less constant voltage up to some maximum short-circuit current.

I would say that a solar cell, or more precisely a series of solar cells are a good example of a current source, because the more of them you add, the higher the voltage you can reach while the current remains the same, which is approaching the second characteristic of a current source:
- the first characteristic being a constant current whether the current source is shorted or whether it is being impeded by some load resistance,
- the second characteristic being a very high source impedance coupled with an very high source voltage which is needed in order to keep pushing the same current through whatever load resistance is trying to impede its current.

Batteries can be an interesting example of both a current and a voltage source. A single cell behaves as a voltage source with an internal series resistance, which can also be thought of as a current source with an internal parallel resistance.

If you wire many cells in parallel, you get closer to an ideal voltage source, but if you wire many cells in series, you get close to an ideal current source, which will deliver something close to short circuit current of a single cell over an arbitrarily wide voltage range.

Of course, this works with many other things, including solar panels. You can build a better approximation of a current source by wiring many bad approximations of current sources in series. If that's not an engineering answer, I don't know what is ;-)