That would be me! 😜
I haven't made them in about 3 years, but keep hinting (promising? threatening?) that I will begin production again soon.
There's a waiting list. It's L O N G . . . Atari Age thread about that here: Intellivision Flashback Controller Adapters - Round 3 (Nurmix)
I have all the components needed to make a lot more, it just comes down to time... and actually motivating myself to start again, since it's quite overwhelming knowing how many people are waiting for them, following up with all those people, actually making the cables, packaging, boxing, shipping. It's a bit like the situation with Joe Z and the LTO Flash.
If you want to make them yourself, there are wiring diagrams here on Atari Age - I don't have the specific thread handy at the moment. There are a number of wires that need to be cut and switched since At Games didn't stick to the same wiring pin-outs as Mattel when they made the Flashback controllers.
As for parts and equipment, you'll need a soldering iron and some soldering skills, a crimping tool, pin insertion tool, and wire cutters. If you want to make adapters for the Intellivision II / Sears Super Video Arcade, you'll also need heat shrink tubing, plastic wire covering conduit, and a heat gun (although if you want to go cheap you could use electrical tape and twist wires together, but I wouldn't recommend that for longevity and reliability).
You'll need db9 cables - the easiest is to buy joystick extension cables (ones for the Sega Genesis work just fine, I use those that are made by Retrobit). You can find them a little cheaper on ebay.
You'll need the 9-pin header / pin housing that plugs into the Intellivision motherboard. I get those from Mouser Electronics (Mouser part # 571-1-87499-6).
And you'll need the pins themselves. The ones I get can be used as crimp-on, but I prefer to solder them to the wires (and I bend the wire ends / cups down a bit before soldering so they slide into the housing more easily). I also get those from Mouser Electronics (Mouser part # 571-1023165).
Here's a video review with installation from GRay Defender: Intellivision Flashback Adapters | Nurmix