I went to a Besiktas game and it was easy, I just walked up to the booth and bought tickets for about £15. They were playing Athletico Madrid in the Europa league. (Loudest game I've ever been to by a country mile)

I went to Fernebahce vs Galatasaray and it was bit more tricky. I bought them of students for £120 each. Well worth it though.

You can get them off the usual ticket sites mind and there were no problems like showing your passport like there is in Italy for example.