Where can I buy really good beer in tokyo?

Hello Nueng!
I believe a very good shop to buy good beer in Tokyo would be Kakuyasu (カクヤス) and you can find it in many places around Tokyo.

Here's a map with many Kakuyasu locations, I hope you can find one near you:
https://www.google.co.jp/search?q=kakuyasu+sake&rlz=1C5CHFA_enJP697JP697&oq=kakuyasu+sake&aqs=chrome..69i57.3476j0j7&sourceid=chrome&ie=UTF-8#q=kakuyasu+sake&rflfq=1&rlha=0&rllag=35663948,139749308,3541&tbm=lcl&tbs=lrf:!2m1!1e3!3sEAE,lf:1,lf_ui:3

Here is Kakuyasu Sake's official website in case you need to take a look at their options:
http://www.kakuyasu.co.jp/

You can find many types of beer and Sake there. I hope this helps!

I believe this may also be useful, here's how the shop looks like:

https://books.google.co.jp/books?id=jlT5CQAAQBAJ&pg=PT55&lpg=PT55&dq=kakuyasu+sake&source=bl&ots=ZqgDViRAwd&sig=JzH5a80KU7BM6U2SifciSDoP3ck&hl=en&sa=X&ved=0ahUKEwjZlOGd0fDTAhVCmZQKHXXkCsgQ6AEIPjAH#v=onepage&q=kakuyasu%20sake&f=false