Polyester and nylon flat braided lacing cord is available from Alpha Wire in different sizes, either waxed or unwaxed. A 457m reel from RS or Farnell will cost you about £50 though. I still have a small 100m reel of waxed polyester lacing cord that will probably last a life time.
EDIT: Thanks sokoloff, avionics wiring bundles are still laced by hand because it's tried and tested and it's reliable. In fact an avionics lacing knot is quite difficult to tie and to my knowledge nobody has made a machine to tie avionics lacing knots.
Motor repair shops would be another source of lacing cord.