Do a general Web search to check to see if there is already a compiled or collected travaux préparatoires.Yale Law School maintains a list of Collected travaux préparatoires
It's important to be aware that there the UN hosts some travaux préparatoires websites for particular agreements.
If no collected travaux préparatoires is available, try a Secondary Source.
Search for a guide to the travaux préparatoires that you are looking for.
Alternately, another good type of secondary source to start with would be an "article-by-article commentary on a particular agreement" (Pratter 2015).
Example of a commercial publication of travaux préparatoires.
Sometimes, intergovernmental organizations such as the Hague Conference or UNIDROIT will publish in one series travaux préparatoires for all international instruments that these organizations have drafted.
Instead of searching for the name of the treaty and the keywords "travaux préparatoires" as you did in Step 3, try searching the three catalogs for:
keywords: "formal name of conference during which the text of the agreement was negotiated" AND keywords "travaux préparatoires"
Note that these sources are not necessarily comprehensive.
If you are looking for "travaux préparatoires" for a recent treaty, you may find it on the official website for the conference.