That is something we need to create. They do come with some calibration but I suspect they are free-field or diffused.
You might want to talk to Sean Olive - he's got a 5128, and I believe I recall Sam Vafaei telling me that he'd offered to provide compensation data to RTings when they considered a 5128. As a side note, if you haven't talked to Sam about the 5128 yet, it might be wise to - he's raised some reasonable concerns regarding its behavior with sealing in-ear designs.
For a widely-measured and poorly-performing headphone design, an earbud or other non-sealing in-ear design (Earpods, etc) would be a reasonable option that shouldn't be hard to source. Earbuds are essentially inevitably horrifically nonlinear devices, due to being a small driver in a big, harsh world, without the safety of the sealed canal volume to keep them comfortable.
Great to see you looking into headphone testing, by the way! I recall talking to you about it quite a while back, but I lost track of where we were in the anarchy that was my last year.
Edit:
If I get the unit you are considering purchase of, and used it to make a great headphone from measuring on that rig, much of the greatness is gone if my pinna is different and/or my ear canal has different peaks due to different shapes.
Please bear in mind here, subjective timbre/frequency response of headphones is equal to eardrum response minus the appropriate listener HRTF - in a world where headphone eardrum response was absolutely constant against user HRTF variation, this would indeed introduce substantial variation in subjective timbre; but do we live in that world?