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. Each is a multi-volume, looseleaf (therefore regularly updated) service providing English translations (not therefore an official text - but a "proper" translation) providing worldwide coverage.
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: