Mathematica ExtraPackage

You can easily get the path to any installed package file using FindFile, for example:

FindFile["Notation`"]

"C:\\Program Files\\Wolfram Research\\Mathematica\\11.3\\AddOns\\Packages\\Notation\\Kernel\\init.m"

If the package isn't installed, you could search it in several places on the Internet. Don't forget about this site, old Wolfram Library Archive and also brand-new Wolfram Function Repository.

Oh ok, so after some further reading, it seems that doing:

<< Notation`

does the trick. I was trying to do something like

Needs[Utilities`Notation`]

but with no success. I am not very sure where Wolfram takes the package from though?