What type of headphone connector is this?
Searching for a format specification starting with the manufacturer of the product:
From the vendor you mentioned


  Onkyo is offering the ES-HF300 with a 6N oxygen-free copper cable encased in clear elastomer, providing almost perfect conductivity while keeping touch noise to a minimum. Gold-plated MMCX connectors and mini-stereo plug reflect the highest build standards.


A quick read that might explain more about the format and its history reveals an industry specification:
From wikipedia


  They conform to the European CECC 22000 specification.


Using a search engine with the connector format and the device type produces many potential matches for the device in question (MMCX connector and headphones):
From some vendor, this appears to be an example of a viable replacement (by visual inspection).

That should get you going.