Spanish speakers speak fast, but I think there are very good movies with a lot of very natural dialogs.
Una pistola en cada mano
Truman (one of the main characters has an Argentinian accent)
Tres bodas de más
Sometimes I offer my partners activities with TV ads. Here you are a list with five ads with very natural speaking (from easy to hard to understand). If you like it, send me a message and I can send you the transcription.
Little late, but if you're still interested:
El otro lado de la cama
La noche que mi madre mató a mi padre
La cara oculta
Rec (1 y 2)
Mujeres al borde de un ataque de nervios
There's the "Extra" TV series, which was created specifically for this purpose (in fact they speak so slowly that it's still intelligible at 1.5x and 2x speeds), and it's quite amusing.