One place you can find Journal Impact Factor is in Journal Citation Reports, part of Web of Science. You can look up a specific journal or look up journals by title.
In addition, most journals' websites include an impact factor, so if you are looking for the IF of a specific journal, you may find it there. In some cases, however, there is no indicator of the year of the particular journal's IF. That may cause discrepancies between what a researcher reports and what is in the current JCR.
