Hi, Jeffrey. Yes, these were really popular, but for now the only way to get one would be if you can find a used one. His daughter, harpist Becky Nissen, said a few years ago that his family were planning to start production again, but as far as I know nothing ever came of it. There are lots of people who’d like to put this cart back into production, but the rights to the design are with his family.
If you want to try to contact Becky to see about the current status, she’s in Myrtle Beach. She used to be at myrtlebeacharpist.com, but I don’t think that site is current anymore. She is on linkedin (as Rebecca Nissen).