Prepared Thai curries are almost always found in paste, rather than powdered form.
You can find this in most asian markets.
It comes in little cans (tuna size) and larger plastic tubs.
Probably 10 difft varieties, including green.
You add it to taste to coconut milk, fish sauce, etc. There are recipes on the can or use an online one.
OR you can make it homemade, like Yakuta