First of all, welcome to the forum.
Second of all, you can 100% trust Cinemasterpieces for a high quality and well packaged product. His website is like a museum, with posters you sometimes can't find anywhere else. However, Cinemasterpieces is the most expensive retail website you will possibly find on the internet. His posters are typically listed for 5-10 times what they're actually worth. Many people (including myself) on this forum will tell you he's an excellent resource, but unless you're an uninformed millionaire, you can do much better for your dollar.
I'm going to send you a Private Message with my entire list of retail websites. There are many movie poster websites, and then there's eBay. This forum will be your most powerful tool in your collecting adventures, so ask a lot of questions and make a lot of friends!