WaveTheory
100+ Head-Fier
Are there any third party single ended cable readily available?
You have a few options. Your safest option is to use a single 3.5mm TRRS connector on the headphone end, regardless of whether you need balanced or single-ended on the amp end. You can get a balanced TRRS cable like this:
https://www.amazon.com/gp/product/B07QKMKPM8/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1
and then adapt it to a single-ended amp with an adapter like this:
https://www.amazon.com/6-35mm-Headphone-Earphone-Adapter-Balanced/dp/B0828LSX54 for 6.3mm
or
https://www.amazon.com/Female-Headphone-Earphone-Adapter-Balanced/dp/B0828KKVZ8/ for 3.5mm.
You can also use Hart Audio Cables' system with HC-2 headphone cable:
https://hartaudiocables.com/collections/headphone-cables/products/hc-2-3-5mm-trrs
and then get whatever interconnect type(s) work for you.
A friend on HiFiGuides Forum says that Apos Flow cables for HE-R9 or Deva Pro are a single 3.5mm TRRS on the headphone end, even though the pic on the page does not match that:
https://apos.audio/collections/apos...low-headphone-cable-for-hifiman-he-r9-r10-r10
Happy hunting!