Here is the link to the Habst clock cables: http://habst.de/digital-xlr-rca-bnc.html You can email the proprietor, Daniel, through the web contact form for more information.
As you have read on this thread and possibly elsewhere, clock cables affect the performance and sound quality of the master clock. The Habst cable is an excellent choice, but there are other options as well. I can personally vouch for the Habst cables as I use them in my system.
People have generally found the shortest possible length is preferable for clock cables. A major advantage of the Habst cable is that it can be grounded to an unused BNC output on the REF 10, which significantly improves sound quality. I ground mine to a Synergistic Research Active Ground Block for even better results. You could probably do the same with your Entreq.
BTW, getting more personal. What do you think of golden ear guru 'austintop'?.