I'm not a movie expert, but under "press kits" under movie memorabilia here on ebay, I found over 10,000 listings.

To get a movie press kit, is pretty easy, but you asked about how rare they are.  I am sure that for some popular and older movies, they would be uncommon and perhaps more expensive, but I know theatres made plenty for a long time to supply newspapers, radio and television stations, movie critics, and various show biz types. 

Press kits often contain a half dozen or more 8x10 glossy photos from the movie that newspapers could put in their reviews, plot summaries, and a list of stars, director, and so forth.  I believe they often came with paragraphs full of high praise for the film that lazy reviewers could use.

The first Titanic movie was made just months after the Titanic sunk.  It was a silent film called SAVED FROM THE TITANIC and it starred a women who actually escaped the sinking liner in the first launched lifeboat.  See details:

 http://en.wikipedia.org/wiki/Saved_From_the_Titanic

The studio (in New Jersey) burned just 2 years later, and every known copy of the film went up in smoke.  The only images we have of the film today are from the press kit that was sent out to promote it. 

If you love a movie, the press kit is fun to have, and the 8x10s look impressive in frames on your wall.  I hope this helps.

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