The best place to find zines and add them to your personal collection is often a local bookstore or art space. Beyond that, the following list is a good place to start and should get you started on your zine collection journey.
Kersplebedeb - This site has a great selection of counter-culture zines for browsing, purchase, and inspiration.
Library of Congress Zine Browser - Library of Congress database that can be searched for zine materials to borrow or preview online.
Quimby's - This site sells zines and will curate small collections for you after you set a dollar amount that you are willing to spend.
Behind the Zines - This zine distro is a great resource and can often link to other good distros as well.
Microcosm Publishing - Zine distro that also sells everything related to zines - stickers, patches, artwork, supplies, merch, etc.