We love Thai iced teas stateside and were pleased to learn that they actually are popular in Thailand as well (unlike some other of our favorite Thai dishes). Thai Iced teas are called “Chaa Yen” in Thai which literally means “Tea Iced.” Haphazardly we found out that by ordering a cha yen flavored smoothie at an Au Bon Pain outside the Grand Palace (not our finest culinary moment).
A typical Chaa Yen is an uber-sweet, red, spiced Thai tea with condensed milk. The best Thai iced tea we had was in Chinatown in Bangkok from a mobile coffee stand called Coffee Cop on Rachawongse. The proprietor could not be nicer and the Chaa Yen was amazing! Back home, we have tried to recreate the recipe, with varying success. Thai tea is somewhat difficult to get in the US, so we have had some luck using Vanilla-flavored Rooibos instead.
Eating The World 
You can buy Thai tea even in the US 😉 For example there http://www.thaiteaset.com/