Japan produces some of the highest quality matcha green tea powder. It has favourable geographical and climatic conditions, and the know-how to produce some of the highest quality matcha teas.
If you live outside of Japan, you may be able to buy matcha green tea powder at Asian grocery stores. Choices may be limited, depending on where you live.
An alternative option is to buy matcha online from reputable sellers such as American Tea Room and Amazon, that ship worldwide. You can find a wide selection of matcha for various uses and special matcha accessories.
American Tea Room offers drinking matcha and other types of tea here.