I know it is not easy. I can’t give you good advice. Here are just some ideas.
Twitter or Blog
You can go to a concert and start talking to someone you don’t know.
But unfortunately many of the concertgoers in Japan are shy. It is probably due to the national culture. Japanese people in general rarely talk with strangers who are sitting next to them.
Instead of talking to someone about the concert, Japanese audience post a short comment on twitter or write a relatively long review for their own blog.
Classical music fans in Tokyo often (not always but most of the time) go to concerts alone. They prefer online communication either on twitter or blog. Some of them use Facebook for more private communications which are not related to classical music because their friends are usually not interested in classical music.
Eventually after some months or years of online communication on twitter or blog with other concertgoers, they sometimes decide to organize a small get-together lunch, coffee, dinner or drink.
Why don’t you start twitter and follow Japanese classical music fans or start a classical music review blog of your own?
If you can write in Japanese, that would be great, but it is very difficult for many of you. I don't know how English twitter posts and blogs can be useful in communicating with classical music lovers in Japan. You can try it though.
Classical Music Bar
This is my recent discovery.
I went to this bar in Tokyo four times and enjoyed listening to music from the speaker and talking with new people. One day I met a pianist and ended up going to her concert the following month.
The bar is on the second floor of an old building in Meguro, Tokyo.
You can still talk to someone you don’t know at the concert.
Some people (including me, of course) are happy to have a conversation with travelers or expats who decided to come to classical music concerts in Tokyo.
If you are there for a concert of your favorite artist, then, don't hesitate to talk with someone! it's a perfect chance to meet someone who is also a fan of that artist.
Some classical music fans in Japan can actually speak English and are willing to communicate with someone who knows different things.
Perhaps you will meet someone who loves both classical music and travelling.
I will keep my fingers crossed for you!