The best approach to purchasing movie posters is to ask an employee or manager at your local movie theater. Some theaters will allow their employees to take home posters after the movies have left the theater and will extend the opportunity to their customers. It really depends on the theater manager's policy, which is why you should consult the help of an employee first. Some movie theaters will not allow anyone to take posters, while others sell them right in the lobby.
If you cannot buy an original movie poster from your local theater, you can always try to find it on the Internet. Movie theater employees will often take movie posters and sell them on Internet auction sites to get extra cash. However, as with all Internet auctions, you will have to pay for shipping fees and may discover that the item is not in the condition you expected.
Another way to purchase movie posters is to find a reprint version. While they lack the large size and are not as rare as original posters, they are generally very inexpensive. You can find several poster stores on the Internet, including AllPosters, MovieGoods and MoviePosterShop.