Where should I download libraries for modules through composer?
Install Composer Manager, then you won't need to think about it:


  Composer expects each project (Drupal installation, in our case) to have a single composer.json file listing all required packages. Composer Manager allows each contributed module to ship with its own composer.json file, listing the module-specific requirements. It then merges the requirements of all found modules into the consolidated composer.json file. This results in a single vendor/ directory shared across all modules which prevents code duplication and version mismatches.