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.
To access reliable English translations of national constitutions online try:
Available on the free web and accessible to all:
For more about books on constitutionalism generally, please visit our separate guide
The following guides each have a pages on both Indigenous peoples and consitutional matters (among others).