Can power outlet be taken as a current source?
Both 'current source' and 'voltage source' as presented in your course in electronics are idealisations. They are theoretical entities we never meet in practice. But they are often good approximations to what we do meet, within a certain range.
The difference between them is the output impedance. A voltage source has ideally a zero output impedance. In practice, any source with a small output impedance is well approximated by a voltage source. Its output current can vary from zero to a large output, while its voltage changes by only a small fraction. A current source has an ideally infinite output impedance. The voltage will vary from zero to a large value, while its current changes little.
The power outlet on your wall is best approximated as a voltage source, for most of the things you want to do with it, like connect lighting, heating, motor loads to it. Its output impedance is very low. Within the range of current that it is intended to supply, the voltage drop is small.