I went through this a while back. Spent a lot of time researching.
As i understand it, ideally, you’re hoping to find an
adapter. Along the lines of this (from the Sound Skulptor kit you referenced):

After spending hours looking for essentially the same thing, I concluded that that’s a custom-machined part. You’ll find plenty of off-the-shelf adapters with 4mm shafts and 6mm collars (intended to adapt a drill chuck to a motor shaft), but I don’t think you’ll find a 3mm or 1/8 shaft with a 6mm or 1/4 collar. I even reached out to a couple of industrial supply houses with no luck.
In the end, I had just enough room in the build i was working on to use a
reducing shaft coupler like the ones you referenced on Alibaba, along with a separate 1/8 shaft I cut for my needs:

The challenge with that solution is that it requires a little extra room vs a true adapter like those shown in the Sound Skulptor site. If you have the luxury of adequate room though, the coupler solution works nicely.
If you do actually find an off-the-shelf adaptor (and I'm not optimistic you will) please share. Good luck.