We went last year and the only place that we were able to find anything close to a tiki mug was at the Potato Head in Seminyak. It's more like a beach nightclub/concert venue, but they have a tiki vibe and tiki drinks. They also had and sold these carved wood "mugs". I think they're made out of teak wood and are kind of like a wooden beer koozie that a glass cup was placed inside. It has the name of the restaurant stamped on the bottom. They had two sizes, a tall one and a short one. When we were there they only had one short one left and I managed to get it.
I imagine they have gotten more in stock since then. It sounded like they started selling them because people kept stealing them :)