Check your local video store. Video stores get new movie posters all the time for upcoming releases. They will probably be happy to give you older posters that they take down to make room for the new ones. Ask the manager to hold posters for you or to let you look at the ones they have stored.
Visit your local movie theater. Like the video store, the theater changes its posters frequently. Ask the manager to save them for you.
Visit antique fairs and stores. If you are looking for classic movie posters or originals, take a trip to the local antique mall. Film memorabilia is popular so they probably have something on hand.
Shop online. There are many online websites that sell movie posters. You'll find a lot of selection but you'll have to pay for the posters and shipping.
Bid on eBay. eBay is a collector's paradise. Many people buy and sell movie posters online through eBay and other bidding sites.
Attend movie premieres in your town. Local theaters often have special showings for new blockbuster movies where they give out hats, posters and other swag.