In the UK we don't seem to have many options for balanced MMCX cables. It also doesn't help that the 4.4mm standard is very new. The Fiio cable from amazon looks good, but you'll need a good adapter, and most certainly a multimeter to check the connections because if it's off you can short the ZX300, which would be very painful.
I ordered a cable from Surf Cables in the US, all in with delivery less than £100 - when it arrives I'll probably review it, but otherwise you could explore the avenue of buying the Fiio cable, cutting it, and re-terminating it in a 4.4mm cable - that'd be the cheapest option but with a bit of hassle attached.