You can get tickets from the Sevilla website, as mentioned, from about a week before the match. Tickets prices are banded dependent on the opposition and for Real Madrid will be the same as for Barcelona.

I went to a Sevilla v Real Madrid game and bought tickets from the stadium the day before the game. Touts were saying only the two highest prices tickets were still available. Not true, I bought 4 seated together, lowest prices, behind the goals (towards the corner and higher up).

Been to 3 matches in Seville and, along with Athletic Club del Bilbao, best atmosphere in Spanish League.