Oxford law students should follow the guidance of OSCOLA.
This names the sources of the first rank as:
Where the treaty has not been published in one of the above, try next the
For post-1960 treaties not yet published in any of above, use the International Legal Materials (ILM) citation.
Other abbreviations you may encounter in English language texts, and where to find them online:
Australian Treaty Series (ATS)
Canadian Treaty Series (CTS)
European Treaty Series (ETS/CETS) European Treaty Series between 1949 & 2003 (ETS No. 001 to No.193 inc.) Council of Europe treaties
were designated ETS. Since 2004 both these and subsequent treaties should be cited as CETS. The ETS form is of course still found in older publications.
Indian Treaty Series (INTSer)
New Zealand Treaty Series (NZTS)
Treaties and Other International Acts Series (TIAS) United States of America
For countries in the Anglo-American Legal Tradition it is quite common to have access to both an official/government website, and one provided by the national Legal Information Institute.
If the treaty is "historical" (concluded between 1648 and 1919.) with a citation including CTS
If the citation includes LNTS or UNTS
If the citation includes UKTS
If the citation includes ILM