Hello all im looking to find a mandrel for finishing arrow and spear sockets, can anyone give me advice on what size punch u should use to make this??

first spear and arrow head sockets are very different in size. If you can do a round taper you can make your own to fit in your harder hole. I have a pair of tongs that one leg has the mandrel built in to the tongs. 

The diameter of the arrow or spear shaft will determine the size of the mandrel.  There are several good vidios on utube that will help you.

A mechanics type roller head pry bar makes a good mandrel. They come in many sizes, are already tapered a little, and the steel is tough. You can find old ones in pawn shops and flea market type places for a couple of dollars. You can forge a better taper or just grind it down to your needs. I have seen these from one inch diameter down to 3/8 or a little smaller. Forge them to fit the hardy hole or cut them off and hold them in your vise.
Hope this helps some.

I have seen one that your talking about Chuck, its 50 dollars LOL, xxxx home hardware, i was thinking a tapered  punch or a center punch would do but finding one large enough is gonna be a problem in my local area, like in this video here http://youtu.be/RCuetibSwOg

 

I pick up bullpins for aligning structural steel and forge the fat end to fit the hardy hole.  I've got them in a number of sizes.

A good "start taper mandrel" for arrows is an old morse taper drill, using the morse taper side- the matelial is usually a good tool steel to boot, and all of this at minimal price!