For cool growing Highland Nepenthes I use pure live sphagnum, packed LOOSELY into a mesh orchid pot this allows the moss to grow out of the holes in the pots and allows the roots to get lots of cool air. Dead / dried LFS from Home Depot or Lowes (or ebay) is pretty much just as good, so long as it's loosely loaded into the pots.
When you get to 6"+ bigger pots it's a good idea to mix in fine orchid bark, perlite, APS, pumice or some other stuff to open up the mix and keep it from becoming too dense. Dense media will likely kill the roots on most HL Nepenthes, they like lots of moisture but also plenty of fresh air exchange in the root area. Lowland Nepenthes (warm growers) are quite often from swampy areas and will put up with dense wet media like peat & sand or larger pots of pure LFS, but still it isn't ideal.