The time has come for me to re-do my dropper post outer cable, picked up 3 of these Clarks gear cables from Halfords – £1.99 gets you an outer, inner and 6 spare ferrules as well as the 2 already fitted to the outer. https://www.halfords.com/cycling/bike-parts/gears-drivetrain-cables/clarks-universal-bike-gear-cable
OK they’re not SP41 outers but £6 for three full sets of gear cables and more ferrules than I’ll use in a few years. If you’ve got a full length rear mech run on an XL size bike they may not be long enough but plenty long enough for droppers.
CRC have the same but you can’t nip down the road to get them. https://www.chainreactioncycles.com/clarks-universal-gear-cable/rp-prod47684?mx=a
