Du-bro #DUB2144. This is heat shrink tubing that has been designed to shrink up to 1/2 of its original size with the use of a heat gun, lighter, soldering iron, etc. It's ideal for insulating electrical splices, and covering electrical plugs and connectors, plus it has many other uses. They come packaged in 3" lengths. They have a 1/16" diameter, is blue in color, and come 4/pkg.
Great-Planes #GPMGPMM1058. Neater and faster than electrician's tape, Great Planes Heat Shrink Tubing is also more dependable. Won't unwrap like tape, because it's a tube that you slip over connections and shrink to a professional-looking, non-slip grip. These are available in popular sizes, and color coded for easy reference.