In-store only. Can't find an online link. Sam Coco's has pallets of Fever Tree 200ml varieties in 24 packs for $5.99. That's cheaper than they usually go for a 4 pack. The catch? They're past the best before date. Glass bottles though. Tasted great to me.
I saw Indian Tonic Water, Mediterranean Tonic Water and Ginger Ale. There may be others.
To round out your g&t's stock up on lemons for 9c/kg while you're there.