Looking for 0€ in Madrid

Hey guys, I’m a tourist looking to find a 0€ bill for my little brother. Currently at the Seville train station heading to Madrid and staying by the airport. I’ve visited a few souvenir shops and tobacco places and couldn’t find any. Does anyone have recommendation?

Edit: is there real and fake ones? How to tell the difference? And how much do they usually costs? I see online it’s around 6€-7€