You can literally build one. Here's a guide on doing it.
If you cannot find a header you can snip off a DVI-M1 header and with a Dremel or a small hammer "coax" the shield around the pins to agree with the mating end of an ADC. At least that's going to be "good enough". You can also do stabbing with 20 AWG and then epoxy pour, solder DVI stabs, and then pour again to make a "good enough" cable.
The thing is this cable itself is passive. There's not special circuits, power, or anything inside the cable. In general, ADC is just DVI + USB + Power for your monitor, that's it. It's just those wires all bundled up into a single cable. And technically speaking, if you're handy with desoldering, you can take a Cinema Display and do a breakout to DVI, USB, and a barrel plug and just ignore the bake your own cable. Or if you're going to a modern monitor from an old Mac, you don't need to bring power and USB with you, you just need to break out the DVI that's coming out of the video card.
The thing is, getting into vintage computing is a challenge. And likely these folks are hitting a new challenge to them, that for folks who've been doing this for a while have just come to embrace. Learning to solder, learning digital electronics, and lots of patience. Specifically to this group, it looks like they need a solution to the G4 Cube. There's multiple video cards out there that will fit into the machine, the Apple Radeon 7500 is one that would provide VGA ports that can go to modern monitors.
But I think what gets me is this part of their petition.
The ability to service and maintain our technology not only extends its lifespan but also reduces electronic waste. By enabling vintage Apple Macs to connect with modern monitors through the Belkin F2E9142-WHT ADC to DVI cable, we can prevent unnecessary disposal of these iconic machines.
Getting into vintage computing or arcade boxes or whatever, yeah "the ability to service and maintain" is paramount. But the onus is on us, the people doing the collecting, to service and maintain. I get that the Belkin cable provides a nice easy solution but ultimately, not to sound corny, we do these things not because they are easy but because they are hard to do.
And also, as I mentioned, this cable is passive so it's fairly easy compared to say the Nintendo GameCube's Component Video cable that had an active circuit inside the cable that took a lot of effort to eventually reverse engineer. But the point being is that vintage digital collecting is a very rough thing to get into, it takes a lot of love and really getting that enjoyment out of very complex challenges. It's one of those "I love it, but I don't recommend it" kind of things.
EDIT: Also I think this part of the article hits best.
As always, you should also consider the fact that Apple has made many proprietary connectors in its day, some of which are easier to find than others. There's lots of iPhone Lightning Adapters out there these days, but who knows where we'll be in 30 years.
Yeah, this is why open standards and the right to repair is really important. And one of the main things about FOSS and the open source community is this really important thing to remember:
Everyday, more of your life and technology will intersect. We are living in an age where Internet access and technology to access it is as crucial as having a mailbox for mail to get to. Proprietary solutions, closed standards, and planned obsolescence are modern ways to gatekeep your ability to freely access this new modern society. We are getting closer everyday where this technology dictates if you can or cannot participate in society. It is vital to have as the bedrock of this technology, the principal of free access to all, so that everyone can freely participate in this society we are growing into.