Scroll down to see all collections currently available or use the Search box to type in what you're looking for (this can include words found in the collection name or description, institution name or a category. Check the optional boxes if you want the results to only show LUNA collections or Https collection. The Featured option will result in collections that have broad appeal.
Click on any of the results to expose information about the collections, including the Share URL which you will need to add a collection to your LUNA instance.
At the bottom of the page, you can see that there are three ways in which you can browse or search for available collections: Collections, Categories or Institutions. Click on the bar to expose the page.