I bought one of these https://www.tindie.com/products/TAUTIC/max31855-thermocouple-to-digital-converter/ to use with a thermocouple type K. It looks exactly the same of the Adafruid product.
I am wondering if I really need a level shifter to use it with an Arduino or if I can connect it directly.
Your comments will be appreciated.
From your link:
Note: You can use it with your Arduino or other 5v i/o logic, you'll just need some form of level translator or resistor divider.
So, if you have an arduino that runs at 3.3V, you can use it directly, otherwise you'll need to make some kind of adjustment.
Thanks. I also noted that but the Arduino due is running at 5 V and there is a 3.3 V available. This is why I am confused. Should I use the 3.3 V from the Arduino to power up the board and connect the output to the Arduino pins?
All you need to make a simple 5V to 3.3V voltage divider, is 3.6Kohm and 1.8Kohm resistors 
Where R1 is the 1.8Kohm resistor and R2 the 3.6Kohm
As I said, there is a 3.3V output on the Arduino Due. Why should I not use it instead and get rid of the voltage divider and/or the level shifter?
Sorry I didn't see you had an Arduino Due. So you use the 3.3V output of your Arduino to power that board, and you can directly connect the IO without voltage divider.
The Arduino Due board runs at 3.3V. The maximum voltage that the I/O pins can tolerate is 3.3V
Guix, Thanks. So if I understand correctly, I need the level shifter to interface the little board with the I/O of the Arduino being these at 5V. Correct?
I edited my post: you don't need anything else
But, 16$ for such a little thing!
I agree but what else to use for measuring temperature with a thermocouple. I tried a self made voltage amplifier but the output was really bed.