NIE Card and Empadronamiento (Barcelona)

Hi all
I am British and I have had an NIE number for years now printed on a document and stamped. This is all good and well. But do I need to get (or rather does it even exist) an NIE card- like an ID card.

If the answer is yes, how would I go about getting this?

Also concerning Empadronamiento- what do you do if you are subletting a room, thus not mentioned on a contract....

Thanks in advance of any advice anyone can give me!