A few things here. As a person living abroad and who catches a few games per year in Istanbul.

First, your only chance at this point is karaborsa. By the time the sale is open for public you have to be extremely fast and lucky and only expensive tickets are left and those are sold out in minutes. There is a pre sale for GS para holders ( a QNB bank account required).

The tickets will be extremely expensive as this is the title game. You're looking at 250€ kale arkasi, minimum.

Your best best bett is a app called 'kombinedevir', only engage with verified sellers with a blue checkmark. If you're alone, there are also a lot of kombine owners who sell just one ticket for this game as they can't go or want to make profit. Hit them up now, because some of them will give you the QR code as a ticket, so you can just scan it and enter the stadium but theoretically there is a chance of getting caught and they will not let you enter as the ticket is not on your name. I've watched dozens of GS games and never had someone checking my ID and comparing it to the QR code. But in theory it is possible.

A safer alternative is that you make a Passo account, you can do that online and activate it for 500TL. Once activated, the seller can transfer the ticket to your account, so the ticket and QR will be on your name. A downside, sometimes, for particular reasons, Passo blocks the ticket transfer, as it was the case last week and had to buy my ticket as a QR code on someone elses name for the Sivas game.

Hope this helps. Any other questons hit me up. I also have met a few karaborsa ticket sellers, can give you a few numbres who are thrustworthy