I have just used cotton braid insulated wire which is about 3mm overall diameter so works OK on 1/2 to 1/4 scale engines, have not had any problems with the spark trying to jump. This looks like the 7 or 8mm braided HT wire. If you just want plain black or red then you can use leads for electrical testers or S/S sell it by the length. Anything looks better than the steel braided stuff on some of the cheap ignitions!
For connectors I use the flat brass type that clip over the top of the plug, solder wire to that and a bit of heat shrink to tidy it up. S/S do clips for various size plugs. S/S Machine & Engineering, LLC
Depending on the size of your plug there are boots available to look like automotive ends. Roy at S/S has high voltage wire starting at 0.100 inch diameter at 10,000 volt insulation. Then as the size goes up so does the voltage.