Browse current physical magazines on the periodicals shelf on the main floor. The overhead scanner in the lobby is an easy way to make a scan and send the image to your email. There are also donated magazines available in the Writing Center. The ads in those magazines can be ripped out or copied.
https://southdakota.overdrive.com/library/magazine-room Brookings Public library has full digital issues of current magazines available for check-out online. You can also browse a few pages of a magazine without having an account or logging in. All SDSU students are eligible to sign up for a public library card, which can be done online at https://www.brookingslibrary.org/how_do_i/apply_for_library_card.php
https://repository.duke.edu/dc/adaccess Duke University maintains a repository of advertisements from 1911-1955. Search by product, year, or company, among other limiters.
Academic Search Premier Search for a magazine title like "People Magazine" and look at the PDF full text versions of various articles in the result list. Some of these sources include the ads as they appear in the print version, especially ones entitled "People Picks."
ARTstor Enter "artsubject:(advertising)" into the search box and hit enter. This search will bring up several thousand advertisements from the 20th century, including ones by Kodak, WonderBread, and cigarette companies.