Here's a quick recipe for making your own maccha aisu! From Japanese Mom's Table:

2 cups (470ml) Vanilla ice cream
4 tbs maccha (Japanese green tea powder)
2 tbs sugar
2 tbs hot water

1. Prepare all ingredients.
2. Make maccha paste by mixing maccha and sugar. Pour in hot water and gather. This way prevents it from becoming lumpy.
3. Mix ice cream and maccha paste with blender.
4. Serve maccha ice cream cool.

If you would prefer to make maccha aisu from scratch, try these recipes: