What's the cheapest and most reliable way to buy El Clasico tickets?

I've been doing some research for ticket prices for El Clasico in March 2019 in Madrid. I've seen mixed opinions on whether it's best to wait until the 'official' tickets are released as they may be cheaper but not guaranteed to get one or buy them from a reseller like StubHub or Viagogo. Are... show more I've been doing some research for ticket prices for El Clasico in March 2019 in Madrid. I've seen mixed opinions on whether it's best to wait until the 'official' tickets are released as they may be cheaper but not guaranteed to get one or buy them from a reseller like StubHub or Viagogo.

Are the prices significantly different? And when do Real Madrid release the tickets themselves?

Thanks!