How can I make Turkish dondurma?
Salep is essentially glucomannan; you can subsitutue Salep with Konjac GM (not konjac flour as it might impart a fishy flavor). And this is the key ingredient in Dondurma.

When it comes to mastic, you can try to omit it as it's mainly for flavor. Not all Turkish Dondurma is with mastic; you could simply use vanilla...

Note: I’ve made Dondurma simply like this, and it turned out almost as authentic as it was in Turkey.

Dry blend the sugar (10% of milk by weight) and KGM (0.6% of milk by weight) and whisk into cold milk. 

Keep it in the fridge for at least 30 minutes. 

Boil and simmer for 15 minutes.

In a stand mixer, with the paddle attachment beat the hot mixture for 20+ and until it cools down to room temperature.

Transfer to a bowl and put it into the freezer, every 30 minutes take out and whisky a bit, until it’s quite solid. 

Keep it in the freezer for another 12 hours. 
And serve.