This can be in reference books, phone calls, or websites. Generally LC likes to have three reference sources cited. And if you do all the necessary research on your end it will speed up LC's review of the headings. There are lots of suggested reference sources in the Subject Cataloging Manual, but you are not limited to these. You must also remember to make full use of the WWW in your research.
Many references are on the Web.
Also, check company homepages and e- journals. Use the telephone if you need to verify local data. Cite this call in a 670 field. If you check sources that don't have information on the term, cite them in 675 so that the editorial review people will not go back and check them again. Also, if another library adds to your heading in the future, they will know what sources you already checked.