Here is a GREAT recipe for Iced Green Tea Lattes. You can order the matcha powder online or check with your local coffee/tea shops. Some speciality shops carry it.
1/2 teaspoon matcha green tea powder
2 ounces hot water (but not boiling)
2 teaspoons sugar (or artificial sweetener)
4-5 ounces milk
Lots of ice cubes
Mix 1/2 tsp green tea powder in 2 ounces of hot water in a bowl. Stir with a whisk until completely dissolved.
Stir in 2 teaspoons of sugar or sugar substitute (can adjust to taste).
Add in 4-5 ounces of milk and stir.
You can also add a teaspoon of your favorite flavor syrup. If you liked Starbuck's Iced Green Tea Lattes, they used a melon flavor syrup (which has now been discontinued) but that is the flavor that gave their lattes a unique taste. There's even petition on facebook to bring back the melon syrup.
Personally, I think it's a bit too sweet with the syrup, the sugar alone is sweet enough.