Antex HP40 iron:
HP40 replacement tip, #B240030 :
Earlier, I had posted that solder iron tips retained by a screw were generally junk, but I stand corrected. These Antex HP series look very good. Also, the tip in the link looks to be the ideal shape for guitar electronics work.
If this size of tip is the one that comes with the iron, you likely won't need another size!