I can't tell you where you might get those connectors, but I can recommend a pure copper cable that has the 2.5mm connectors you are looking for.
Hartaudio cables in the US makes really nice cables and they are not overly expensive. While they can custom make the proprietary "notch" with the 2.5mm connector, I have found that it isn't necessary. I (accidentally) ordered one with just the regular 2.5mm connector (and a 3.5mm SE and 4.4mm balanced source connector) and they work fantastically, and more importantly they sound great with it. With the modular connector of your choice, I think it came out to $55 total. It shipped and arrived very quickly (I think they are based in Texas IIRC). I know this isn't what you asked for, but at least you have alternatives if you don't want to mess with it.
BTW! I think they quoted me an extra $10 or something like that to make the custom ends for the HD700.
P.S. I also did some research on this, and for the cup connector side you can actually get mono 2.5mm or stereo 2.5mm safely. The phones are wired as balanced, so you won't be shorting out any grounds with either option.