The official text of a country's constitution is self-evidently in the official language(s) of that nation. To find these it is probably best to go to the official government website of the country you are interested in.
However, there are various ways to access English versions of national constitutions electonically:
Finding the text in print
In the collection with shelf mark General you can find Constitutions of the countries of the world General 510 C759 & Constitutions of dependencies and territories General 510 C760. The physical volumes ceased to be updated in 2020, as it is now available online. The Law Library has kept the previously superseded pages: should you wish to see these, please email firstname.lastname@example.org in advance of your visit.
For more about books on constitutionalism generally, please visit our separate guide
For more on the constitution of a particular country, our jurisdiction guide may be useful