Schweppes tonic is manufactured and distributed in Turkey by Coca Cola TR - they do not produce Slimline tonic. The Schweppes tonic made in Turkey is loaded with sugar, almost double that of UK Schweppes tonic.
Most of the stuff you see locally will likely have come from Greece or Cyprus in someones boot or a suitcase - that said, there must be an independent importer somewhere as I've seen it in several bars in Istanbul.
JF