Hypercolor was manufactured by Generra. Generra was taken over by Public Clothing Company. According to their website "In 2002 PCC acquired all rights to Generra and is now gearing up for a major relaunch in spring 2004." So their site is a little out of date...
There's some information
on Wikipedia. It's somewhat popular (mostly as a retro thing I think) so there's a chance of revival. It may not be worth the marketing and development dollars for what would essentially be an even shorter lived fad than last time though.
You can at least buy thermochromatic pigment. I think you can buy it in screen-printable form.