Chisel mallet

[G S Haydon]

Anything above 12mm, in my experience is best bored out and then pared, even softwood etc. Narex do decent enough mallets via Workshop Heaven, you can then make a decent one when ready.

 

[swb58]

I've never had a 'proper' wooden mallet until quite recently, now I've acquired three. Thankfully they are all different sizes, the biggest is a bit of a monster, capable of some serious beating.
Maybe it's a bad habit, but I've always made do with an odd scrap or lump of wood that's within easy reach. As long as it's comfortable to hold and a decent weight it doesn't matter what it looks like, it's just a variation on a theme.
I think I've got two old favourites, one is a short length of ancient oak post, the other is basically a crudely fashioned lump of hawthorn with part of the branch as a handle. The latter looks quite ridiculous to be honest but it's balanced nicely and works really well.

 

[AJB Temple]

+1 for Thor. I did not know these were being recommended, I just found I preferred them over wooden mallets, which I find unwieldy and clumsy when they are heavy enough. My everyday mortise chisels for framing are ex eBay old chisels (Ward and the like) with metal ring hoops on the hands. I tend to hit this with the copper end of a 2lb or 3lb Thor.