The heads are from Kult of Athena: https://www.kultofathena.com/product/forged-small-spear-javelin-head/
They’re $17.96 each plus a few bucks for shipping. Don’t bother getting them sharpened, they’re good to go as-is for SCA throwing.
Lowe’s also sells a hardwood mop handle without the metal ferrule. It’s already tapered on the end, too.
DO NOT GET THIS.
The reason is that these pre-tapered ones are 1 1/8″ diameter. That’s just too thick and heavy to make a good javelin, in my opinion. The ones with the metal ferrules I’ve linked above are 7/8″ diameter, which is perfect. Buy those.
Once you have the heads and handles, start by sawing off the threaded metal ferrule on the mop handles. I use a chop saw for this, but literally any saw will work.
Once that’s off, you need to taper the end. I use a belt sander for this, which is probably fastest, but you could do it by whittling down the end and then sanding, either by hand-sanding or with an orbital sander. There’s no right way.
Sand them down until the handle fits into the head past the hole in the socket. You want to be at least 1/2″ past that hole. A super-snug fit isn’t necessary – all of mine have a bit of wobble. Set them into place by holding the spearhead socket and pounding it onto the shaft by banging the butt of the shaft on a concrete floor.
Once it’s set, use a drill with a 9/64″ drill bit to drill through the hole in the socket and through the wood, until you hit the other side of the socket – but don’t drill through the other side.
Once you’ve made that hole, take a common 10D or 12D box nail, and cut it off with a hacksaw, leaving 7/8″ of shank below the head. That’s just enough to go into the hole you drilled all the way. Then stick it into the hole in your shaft through the hole in the socket. For a slightly less “nail-like” look, you can pien the nail head a bit with a ball pien hammer.
This will be a little loose, and that’s fine. I haven’t yet lost one during use. As the spearhead loosens up a little on the shaft, it’ll pull against this nail and keep it in place, but if you break a shaft or damage a spearhead, and need to replace it, it’s a simple matter of pulling that nail out with some pliers or prying it up with a blade. It’ll pop right out with minimal effort.
And that’s it – an excellent throwing spear for about $25. These are legal in Æthelmearc, but not necessarily so in other kingdoms, some of which have rules about length, weight, and so forth that I cannot guarantee these meet.