Recall everything you know about the particular “Non Sequitur” comic for which you're searching. Did you see it in a book or a newspaper? Do you remember the subject, publication date or any of the text? The more you remember about the comic, the more you can narrow the search and the likelier you are to realize it when you find the comic you want.
Search for the comic by date, if you can narrow down a reasonable date range. There are a few websites you can use for this purpose. GoComics hosts Non Sequitur comics from February 16, 1992, to the present day; however, only paid users can access comics older than 14 days, and each comic is on a separate page. AMU Reprints hosts Non Sequitur comics beginning from January 1, 1996; go to “Advanced Search,” choose “Non Sequitur” from the “Feature(s) to Search” menu and input a date range to see a list of comics.
Search for the comic by keyword or subject. Under “Advanced Search” at AMU Reprints, choose “Non Sequitur” under “Feature(s) to Search” and input a keyword or keywords. You can also search by category by selecting topics from the “Topic Terms” menu. If you know a date range, you can help narrow your search further by using both a date range and keywords or topics in the same search.
Ask other “Non Sequitur” fans if they can help find the comic you're looking for. For example, you might try posting a discussion in the “Non Sequitur” Facebook group. Other people familiar with “Non Sequitur” might recognize the comic from your description and be able to help you find it, or at least narrow down your search.