Note that "United Nations Convention on Contracts for the International Sale of Goods (1980)" is a Library of Congress subject heading that can be used on TALLONS. Of course, searching by keyword is also possible.
Articles on the CISG can appear in all kinds of law journals. As of March 2019, a keyword search on the name of the Convention returned over 400 results in the Index to Legal Periodicals.
The CISG Advisory Council is a private initiative of international scholars of international sales law. It issues opinions relating to the application and interpretation of the CISG. The opinions would have formally the status of secondary sources, but they have substantial persuasive authority. The opinions can be found at the link given below.