Hey, so here's the deal, a lot of these posters I believe are the property of the artists, I know that for certain in the case of Renato Casaro.
His site is here: http://www.casaro-renato-art.com/MoviePosters/WesternEng.html
I would recommend looking for the artists signature in the poster, and trying to find if they own a website of some sort that you can contact them at. I've only found Casaro's site however, and I don't think that many of those artists are still alive.
Here are a few artists names that I've been able to find:
I wish I could help you out more, but this is a really difficult area.
I'd also suggest a book, "Spaghetti Western Poster Taizen" It has a large collection of poster art from Spaghetti Westerns, however it is in Japanese, it might help you.