Impossible to use my iMac Pro with a 4K monitor

TL;DNR As an Australian, I have no ability to get a 4K@60Hz image from my iMac Pro to my 4K external display.

My external display is a LG31MU97. It's IO is miniDisplayPort, DisplayPort, HDMI 1.4

I want to connect this display to my iMac Pro. Piece of cake! Just buy a USB-C to DisplayPort cable right? Wrong. That cable, though it says 4K on the box, only support 3840×2160 @ 30Hz max. The monitor is 4096×2160@30Hz. I tried the cable anyway, but the iMac Pro literally doesn't recognise that a monitor has been plugged in; it goes black for a second as macs do when a monitor is detected, but no dice. Not even a 1080p signal like my Macbook Air provides.

So I call Apple about this, and they suggest:

• Thunderbolt 3 to Thunderbolt 2 adaptor

(I point out that won't work, as on the box it says "This adapter is not compatible with displays that use DisplayPort or Mini DisplayPort". I've tried with one anyway, no dice)

• USB-C Digital AV Multiport Adapter

(no good as it also maxes out at 3840×2160@30Hz).

• Moshi USB-C to DisplayPort 5K@60Hz Cable

Great! Looks perfect! Unfortunately this is only on the US, UK and Europe sites. No way for me to get it in Australia. I mention this to Apple Support, who try their best for 3 days but have absolutely no way to get it to me. They end up offering me the value of the cable in credit on the Apple store (which they didn't need to do!)

So I've been looking, and there seems to be no earthly way to get this cable! No Australian supplier of Moshi has it, and no one ships it to Australia. I've called my local Apple stores, and they all say they can't get it.

¯_(ツ)_/¯