The YÖS exam can be taken in a few east-european and asian countries as well, you can find a list if you search a bit at the website of YÖS (just google YÖS).

As far as I know there are no websites to prepare for YÖS, only special schools in Turkey self and books available in (mainly Turkish) bookshops. Sometimes you can order them online.

These are the countries YÖS is given in too. From what I know, the exam is at the exact same day and time as in Turkey and just the same. Some countries however, like Arnavutluk (Albania) have different exams that are valid in YÖS too (but they contain MUCH more topics to study than YÖS..)

Kabil (Afganistan), Mezar-ı Şerif (Afganistan), Tiran (Arnavutluk), Bakü (Azerbaycan), Dakka (Bangladeş, Almatı (Kazakistan), Nairobi (Kenya), Bişkek (Kırgızistan), Beyrut (Lübnan), Ãœsküp (Makedonya), Kuala Lumpur (Malezya), Kahire (Mısır), Ulanbator (Moğolistan), Taşkent (Özbekistan), İslamabad (Pakistan), Kazan (Rusya Federasyonu) , Dakar (Senegal), Şam (Suriye), Duşanbe (Tacikistan) ve Amman (Ãœrdün)

I was wondering if it is possible to study in istanbul in english? 

suppose that I passed the YOS exam with good point, is there any university in the turkey to instruct completely in english? 

And if they instruct english then why do I have to pass YOS in turkey?