Make all my own lines use parachord 380 available at michaels craft store, pull out the center core and feed line through. available in all colors and cheap as dirt. Has lasted over a year without significant abrasion
Strength of the sleeve is not critical, when sewn the line takes the load, the purpose of the sleeve is to prevent the thin line from overtightening on itself and shelter it from friction.
the larks heads tighten the knot and carry most of the load preventing the sleeving from slipping so the stitching strength is not of crucial importance either. (but still important)
Anything that fits and has mildew and UV tolerance and decent abrasion is probably pretty good. http://www.kiteforum.com/viewtopic.php?f=1&t=2382752&hilit=+sleeving