To find a citation to an official treaty source, use Treaties in Force, updating it with the tools listed here.
NOTE about the dates covered by the print editions of the Bluebook official sources for U.S. treaties:
U.S.T. and T.I.A.S. are no longer published in print. For the dates covered by the print publications, see the box "Post-1949 official treaty sources" on this page and the Dept. of State, Office of Treaty Affairs page Finding Agreements. PDFs of later T.I.A.S. slip treaties are on the Dept. of State website and in HeinOnline.
If your treaty is not in U.S.T. or T.I.A.S., it might be in one or more of the other official sources listed in Bluebook Rule 21.4.5(a)(i) and (ii). Those other sources are included in the other two boxes on this page, Pre-1950 official treaty sources and Post-1949 official treaty sources. Those lists also include a few more official government publications that, although not listed in the Bluebook, might contain the text or indicate the status of a treaty before it is ratified, e.g., Senate Executive Reports and presidential communications to the Senate.
See also Bluebook & other unofficial sources and Presidential statements, proclamations, etc.