I went to El Clasico last season.
I am not a member and booked flights and hotel and took a gamble at getting tickets because they are usually released in the days before the game.
I think it was on the Friday (one or two days before the game) that the club released around 1,000 tickets for general sale at 9 or 10am UK time. They announced it on the website prior to this.
After 2-3 hours of the site crashing because of the sheer volume of people on the site, I managed to get 2 tickets very close to each other in a good position.
I was lucky. I don't know if the same will happen this year with those released for general sale. But if you are planning to travel then you are taking a gamble, in my case it paid off..