The last time I did mine, I did it online through a friends account with Garanti Bank.
I've just asked him if things have changed since, and he says no, he's done it for a couple of other people in the last few weeks.
If you've got an account with a Turkish bank you can do it online or at the branch.
You can also go into any Fatura Bank branch and do it there. They charge 2 or 3 YTL for each transaction.