You'll better buy one, because chances to get it working with thermalte cabel are pretty small.
I've already tried 3 cables, one that the case has included, 1 from amazon and 1 from Thermaltake Tech Support (got it end of October), each Riser card looks pretty similar to another, i could not spot any differences between them and all these card had same issue. As for me, all this cards belong to single revision, there is no difference, at all.
It didn't worked with 2 setups, once "970gtx+msi z170a" and "1080+asus z170 mark 1", most popular cards from nvidia for the last year and new chipset from intel.
Now i'm forced to buy a riser from another vendor, a working one (regarding feedback from other core p5 owners) cost about 40-70 Euro. Pretty sad that thermaltake cant provide good riser cards, however i'm not upset with that because the case itself is very good for that price.