At the risk of sounding too obvious, the iESL is supposed to have the correct cable included in the box. Have you dug through all the materials? If you know it's missing from the box (or even if you've purchased it second hand) I'd contact iFi through:
1) the general inquiry contact if you bought it second hand to see if they'll sell you the part or
2) the support ticket system if you purchased it new and have it registered
Lastly if you're really impatient like I am, you can buy a HDMI spec cable. I doubt the folks at iFi will be able to tell you which ones immediately available to you have compliant grounds, but a dealer may be able to help. As an example someone at a company like Blue Jeans cables would be able to tell you immediately if the cable is built to spec. *I have no affiliation with Blue Jeans.
I've found iFi's support team very responsive, and that avenue is typically better for a product issue or sales inquiry. Hope that helps you get your cable faster (or found)
[Edited
@iFi audio posted a direct support link just prior, and I corrected Blue Jeans company name]