Roof bars

Just one other point. If you go for a fork mount, which I prefer for a carbon frame, make sure its the right fit for your forks! It was easy when we all had dropouts, but if you have thru axles you need to ensure the rack accepts these, or buy a universal adaptor which allows thru axles to fit a rack designed for dropouts. Again, roofbox.co.uk will keep you right.

The whole point of T-tracks is to achieve compatibility between accessory manufacturers, so as lastboyscout says, any T-track bike rack should fit. I have Cruz aero bars, with Atera T-track, fork mount bike rack. Works fine. However from your link, you are already at the best place to advise - roofbox.co.uk, who I have always found helpful and competitive.

I've also got Whispbars and they will accept any bike carrier that mounts with T-track fixings.

I use Thule ProRide fork mounted ones on mine.

The equivalents to the one you've mentioned are the Thule FreeRide 532 or ProRide 598, but check the roofbox site for other options.

If you absolutely must have the Whispbar one, then try eBay?

You could keep looking or ring up a few places. I got lucky with a mount blanc (ceased trading) holder at one place (he found it out the back).
Otherwise you might be starting from scratch with the whole shooting match.