Here's the story:
On August 13, 2017, Anime News Network announced that Discotek had licensed, among other things, the 1980 film Ashita no Joe (aka Tomorrow's Joe) and would be releasing it on Blu-ray and DVD in Japanese with English subs and in English. The English dub would be from the 2008 DVD release of the film by Tai Seng.
Somewhat confusingly, the Discotek website doesn't have anything more recent than 2015. However, their FB page talks about the release being on March 27, 2018.
This new release can be found on places like Amazon and Blu-ray.com. As for the Tai Seng version, I can't find any instances of the DVD on the internet, but I admittedly didn't search all that hard. If you're really determined to find that version (of which the quality apparently wasn't great), you can probably find it if you search hard enough.