barrel nuts

Don't know if these guys will be of any use but give them a try www.bronzeandbrassfasteners.com.au/

Probably need a minimm order of 5000

Could be worthwhile contacting some fin manufacturers - C3, tectonics, etc and see if they can supply small quantity. Tell them the nuts fell out of their fin and you need replacements.

Try these guys www.surfsailaustralia.com.au/

I know they have reboxing kits for fins that include those suckers, so maybe they sell them seperately as well.

Windforce have some, in a box under the counter.

I got some not along ago.

Also got some brass threaded rod from Bunnings and made some, same as suggested.

Then found it is hard getting them aligned vertically when you glass them in the fin and then found you got to tap the hole in the fin deeper than the nuts after you glass them in, because the bolts stick through the brass nut when tight, deeper in the fin than the nut is deep.

Then decided is was just as easy to buy a new fin, and it meant I got a shiny new fin, which was slightly more exciting than knowing that I had sucessfully fixed a broken old fin.

Or ......if you don't want to go the brass route...

I use acetal rod.
It is similar to the stuff they use in a Nylock nut and you will never drop a screw in the drink even if it is undone.
I use 1/2 inch Acetal rod (12.7mm) cut it to 15.5mm lengths.
I drill the fin head 12.2mm and tap the section of rod in with a mallet, then drill 5mm and run the 1/4 inch tap through.
I've never stripped one out and so long as you don't use the screw to pull the fin into the box you should have no problems.