I couldn't manage to download from OSDN or older sourceforge.jp links either. The whole site is running slow for me but the downloads are still listed so hopefully it is only a temporary issue on their end.
I also found some forks of the program that may be of interest:
https://github.com/clach04/tombo
https://github.com/clach04/puren_tonbo (I think this one is a rebuild from scratch, not usable yet but under active development
)
https://sourceforge.net/projects/tombo-u/
https://sourceforge.net/projects/qtombo/ (Linux only binary, win32 needs to be built
)
I was able to start clach04/tombo and tombo-u on Windows 10.
and I'll just post these links here for posterity:
https://tombo.osdn.jp/En/ - Project homepage
https://web.archive.org/web/20130714120248/http://bluefield.dyndns.o... - Older archive of versions up to 2.0alpha1
(file downloads not archived on the couple crawls I tried
)
http://homepage2.nifty.com/thirami/ - Hirami's old homepage which has unfortunately been unlisted from the wayback machine