I bought a few French films on DVD the other day, all from the fifties and sixties and all superb. I'd recommend them all:
Le Trou - http://www.imdb.com/title/tt0054407/
A prison break set in La Santé prison in Paris, most of the actors were not professionals.
Bob le Flambeur - http://www.imdb.com/title/tt0047892/
A casino heist. The main character Bob is a very suave, aging gambler.
Du rififi chez les hommes - http://www.imdb.com/title/tt0048021/
(also called just 'rififi')
A gangster film and another heist movie.
There is some great slang in these films too: