In another thread on this forum, someone mentioned Intercard srl as a good source for images of Italian Western posters...and I posted that they are a good source to buy actual posters from.
Here's a link to their site:
You have to contact them for details on each poster (they don't list their prices or stock levels on the site). But, I have always had great luck dealing with them.
http://www.moviegoods.com (as someone mentioned above) are OK. I have purchased some really nice posters from them---but, have also ended up getting kind of screwed on some, too. They are not real clear about the condition of their vintage, original posters. However, they do make some nice "Master Print" re-print posters.
http://www.emovieposter.com/imagearchive/search/description/spaghetti+western.html[url]http://www.intercardsrl.com/poster/index.asp are generally good to deal with. All of my Tony Anthony "Stranger" memorablia was purchased through them. I collect mostly b&w 8x10 photos (in addition to posters) and I have purchased a lot of photos from these folks over the years.
eBay is an excellent source for posters!
If you are interested in Japanese posters (for Spaghetti Westerns or Japanese films) I recommend the seller that calls himself cotton_comes_to_tokyo. I've picked up some outstanding items from him in the past.
But, he is just one among many that offer great posters on eBay.