Heat Shrink Tubing
Underwire - Heat Shrink Tubing
Sold as length of 15cm which will tip 4-6 pairs of wires.
Method (from Bra Makers Supply)
After cutting down your wire (using safety glasses), cut your tubing to approximately in the range of 13-18mm long and place on the cut end of your wire with about 3mm overhanging the edge. Use a heat gun to shrink the tube to half its size and it will firmly fit your wire. Then use pliers to pinch flat the extended end of the tubing hanging over the wire, then trim to within 1mm of the wire.