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)

Primos

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.

https://www.youtube.com/playlist?list=PL3YUSDtEsV_qhs5UYewqVsX7x6c_1IldS

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.