You can buy rca to xlr cables from Monoprice. They are quite inexpensive but work fine. Using a set myself between my IceEdge amp and Nuprime preamp.
That's actually where I started. I had a Monoprice RCA-to-XLR cable between an iFi DAC and the Neumann KH120's,but the sound was muffled and rolled-off. Substituted the Stealth PGS cable (RCA) + adapter and the sound became much more clear and open. Not a fair comparison, though, as the Stealth cable is many multiples of the cost of the Monoprice. Many. However, I had it available to me and so I gave it a try. So far the Stealth + adapter realizes the best sound I've had in my rig.