This is after clearing security? I looked and looked for a sales office but could not find one on Saturday; was connecting from Delhi and looking for sales office to see if any upgrades were available.
The TK ticket office is available in the Western part of the International terminal.
If you were coming from Delhi, you would have had to clear entry immigration to access the upper hall, where the ticket offices are located.
If you coming from Istanbul city, you would need to clear primary security check to access the upper hall, where the ticket offices are located. You would not need to clear exit immigration.