Yep there are a few brands around that manufacture fully modular PSU cables, which includes single 8 pin PCIe cables which is what you're after (so there is no additional pig-tail 8 pin).
CableMod are popular and OcUK sell them amongst other places too, but they are quite expensive imo. There's a fairly famous Asian based retail site that sell brands like FormulaMod and Winkool (who I've used several times) that make them a lot cheaper.
I've only just ordered another set for a family member last weekend infact, also for his RM1000x. I got a fully modular cable set from Winkool for £60 delivered - which includes the 24 pin ATX, 8 pin CPU, SATA and 3x PCIe 8 pin cables.