Ballet/ Concert

Is there one website where I could find out what performances ( Ballet/Concerts) are being held in Tokyo and perhaps the nearby regions? At the moment I'm googling at random. Thanks.

Not really. There used to be a magazine decades ago, but I suppose there are too many events today to be listed in one medium. But here are some websites you may want to look into.

Tokyo Time Out (Some of the major attractions suited for tourists)
https://www.timeout.com/tokyo/culture

Ballet Channel
https://balletchannel.jp/category/watch

Concert Square (for classical music)
https://www.concertsquare.jp/concert

I hope these help.

Thank you Uco. The links are helpful!

There are plenty of options, though most only in Japanese :
(use of Google website translation may help)

1. Visit a specific Concert Hall's website (e.g. Suntory Hall, NHK Hall, Kawasaki Muza, etc.)
2. Visit the Orchestra's website (e.g. Tokyo Symphony, NHK Symphony, etc.)
3. Visit the Ticketing websites, e.g.
https://eplus.jp/
https://l-tike.com/
https://t.pia.jp/

Plenty of stuff there, almost every day a classical concert, Opera and Ballet not so frequent.

Another idea:
Amateur Orchestras. Usually of high quality and quite ok to go to, even if you are not a relative ;-)
They usually play in the smaller Halls and City hall Halls.
Google for "Hall" (in Japanese)