I think it is all depending on how you would like to use matcha.  If you are thinking to use it for cooking purposes, super market quality will be abosolutely fine.  However, if you are thinking to make a bowl of tea and drink it as it supposes to, you may not enjoy one from a super market.  I do tea ceremony myself and I usually purchase one from a local tea shop as the shop owner gives me some advice and recommendation within my budget.  Those tea shops were usually run for long time, so you can ask at the information centre.  Good luck with your matcha hunting!!

お茶葉はどこにでもあります。

一番安く買えるのは、supermarketです。

お茶の専門店もありますが、高いです。

色々な種類があるので、少しずつ、色々な種類を買うのもいいと思います。

You can also buy "sweetened Matcha" or "Hojicha". Have you ever been to "無印良品"？

I'd recommend you to visit!

matcha→まっちゃ(抹茶）

Are you learning the Japanese tea ceremony?

あなたは茶道(さどう)をまなんでいますか？
The genuine matcha is very bitter.
本物(ほんもの)のまっちゃはとてもにがいです。
I recommend that you buy “Japanese green tea with matcha”
私はあなたに“まっちゃの入っているお茶”をすすめます。

Please enjoy a nice trip！:-)

You will buy matcha anywhere.

The cheapest shop is supermarket.

My English is mistaken?