Dive shops catering to the Scuba set.
Need to know make and model. Helps to know the diameter and length needed, as well as the style of the latching end of the spear.
In general, there is more interchangeability at the pointy end than at the rear.
Might not be a bad idea to sort out whether you can find a decent mail order supplier, as like as not, you are going that way anyway. Not a huge call for spears here in the colder areas, like there is down in the Southern reaches.
Worst comes, you order in some 17-4 or 15-5 rod of the correct diameter, and carefully file and fit until it works reasonably well. Thread the end to screw a head on, and a sliding ferrule for the line.
If you find a picture of what you need, post it, or PM me. I may have. I inherited a handful of random shafts from a bud that used to have a dive shop. Still have a couple in my stock rack by the lathe.