UN charter requires members to register all treaties. Arrangement of these volumes is by date of registration and not ratification. There are often long delays in registering treaties and some nations refuse to comply with the requirement altogether. Citations are made to volume and page numbers. For example, 574 UNTS 345. Publication is very slow, delay is about 5 years. Indexing can also be sketchy. Sometimes volumes are skipped and published later.
Searching for UN treaties can be tricky. The best approach is to use the online searches to find what is needed.