Your automotive store is a good place too get wiring and connectors too. Make sure you get rosin core solder for electrical connections.
Some hardware stores don't carry electrical wire in all the gauges so sometimes you have to scrounge it. The 18ga wire I used on my 15-91 rewire came from another Singer machine. The connectors came from O'Rielly auto parts. The solder I've had for a long time. The BEST electrical solder used to be sold by Radio Shack before they abandoned the electronics for cell phones and computers.
I used individual wires for the motor with heat shrink and liquid tape for insulation.
Just in case you don't have it, here's a link to my thread about the 15-91:
{
http://www.quiltingboard.com/vintage...r-t205360.html }
Joe