Generally, the best place to start when looking for back issues of a specific periodical is under the "Articles" tab on the Library home page. Here you can see if what you need is available within the Library's resources. Type in the name of the journal you are looking for and any full-text electronic availability as well a link showing our Library catalog's paper holdings (if any) will be shown.
If the issues you want are not available, a librarian can help you find which other libraries might have the older issues you need. Also, you can always try the publisher's website to see if back issues are made available there.