I like google image searches a lot so I did an image search for:
Return of the King Poster - that gave no hits for a Barad-dur poster
Return of the King Poster Barad-dur - that gave no hits for a Barad-dur poster
and then I did an image search for the image Daniel linked to (the cover of The Art of the Return of the King book) to find similar images - that gave no hits for a Barad-dur poster
So, that's a pretty massive amount of info that didn't give any results. You could check poster stores. Quite a few sell movie posters and I used to reference them back in the day for LOTR posters. Be sure to check the WB store. Check whatever Tolkien merch stores you can find.