Do they still exist? In Europe, correct me if I'm wrong, they sort of exist, but in Japan, it's a big question mark. The making of MSX Doujin games seems to have been done with since the last several years.
I found this. It has a price of 300 yen. And released in 2008! Maybe there is still a Doujin convention!
And then I found out they had a website. They had an order option, but it was empty. So maybe that's where SPHERE was distributed? I lost hope.
Hopefully, there's still an MSX Doujin convention. I'll happily go there if one still exists.