I'm sure you are watching eBay, something will probably pop up eventually with enough time. On a side note, do you have an alternate plotting system for navigation? I have an old Garmin running the Bluechart chip (bought used chip off eBay) but I would not rely on 20 year old charts as my primary source for navigation. Channels shift, buoys are moved, new hazards may now be present.
I use Navionics as my primary nav software. For a $15/yr subscription I can run this on multiple devices, including a Samsung Galaxy tablet that I bought for $50 and my phone as a secondary device. Granted I'm not doing longterm cruising so the need for a full chart plotter is not there. Sounds like you may be in the same boat