Buying Match Tickets Abroad (Istanbul)

Hi,

I'm planning to watch Napoli - Juventus(Friday) and Real Madrid - Barcelona(Saturday) matches.
I know just a little informations that how i can take the match tickets from here.
Actually I looked some web sites but the prices are very different. I saw Protected content to Protected content price for same stands.
Also I don't know taking the tickets from here is whether safe or not.

Do you have anny suggestions?

Really Thanks.