i use either gutted std para cord or spectra cord myself the spectra is about what CRK and MT uses for fobs ..........
REI has some cord similar to the spectra bit its not available in black and i use black exclusively for my fobs, a "fob-snob" i suppose lol.
i like fobs on tip up folders to help draw from the pocket FWIW, also good on smaller folders to add some length to the handle.