Thanks. Sadly, I looked into their website and couldn't find a dual 3.5mm male cable, but they do seem to be really well made. Are these OCC made?
The list dual mono 3.5mm as "Type VIII" on their list of options. Rather than list what headphones those are compatible with, they list the headphones that they actually test them with. For Type VIII, that's the Focal Elear. Those connectors should fit anything that takes dual mono 3.5mm, though. I'm not sure if that is what the Sundara takes, but you can email Periapt to find out. For that matter, they would probably be willing to make something with just about any connectors you want if you email them. They are quick to respond to email and, even better, they are very quick in turnaround time (at least compared to other custom makers). Plus, their most expensive option is $88, too. Gotta love that.
OFC is measurably not as good as OCC. However, that is only one consideration. Shielding, braiding, wire gauge, quality of copper, quality of connectors, workmanship, and a a bunch of other small details can change the sound of a cable. OCC is something to consider, but it doesn't make or break a cable. Not like faulty soldering or fragile shielding does.
Periapt is also a good recommendation because, if you don't believe different cables do anything to the sound, these are still well-built, handsome, inexpensive cables that are beautifully supple and flexible. They are delightful to use and a breeze to store, something that cable-makers often seem to overlook. And customer service has been wonderful and prompt.
There are other good custom cable-makers, of course, but Periapt really has their crap together and can't be beat for the price.