HP doesn't sell anything for these plotters anymore. It's strictly a graveyard, NOS and 3rd party supplier situation. I've also used the refill kits in the past.
Turns out to be just another project., the VFD is dim and I'm seeing a Y encoder error, at least. We'll see when I can get it into the lab for a teardown.
On the bright side, when it works, it won't refuse to print because an ink cartridge is outdated.