This answer depends on the age of the chisels. Actually, they are easily made.
If I recall, modern socket chisels (Lie-Nielsen and the like) have sockets that are lathe-bored and are all the same size and shape - this means that the handles are interchangeable.
Older socket chisels (probably pre-war) were either swage-forged or the socket was formed with a seam that was forged. If you run your finger inside, you may sometimes feel the the join. This means that each chisel had an individual socket profile..... it may not be perfectly straight or even round. It doesn't matter if they are a bit irregular.
Sometimes they were heavy duty chisels for framing and the handles tended to be disposable when they split. The user would whittle a new one, just as you're about to do!
If you don't have a lathe, new socket handles are quite easily carved by hand- takes about 20 minutes. Always were by old-school carpenters.
Basically, you'll need some small off-cuts of wood that doesn't split. Slow-grown or tight-grained Ash, Beech etc....... of a suitable length - say, 6 inches or so and of a width to fit your hand.
Plane it square.
If you have the old, broken handle, cut it down the centre axis, to give you a shape for the new one.
If you don't have the old part, cut and shape a piece of card by trial and error that fits into your chisel's socket end. This gives you the shape that you need.
Then cut another three the same and glue each one so that they line up on the end of your blank - or, pencil around the old shape - this will give you a profile on each face to cut to. When this is done, you need to chisel, knife-cut or rasp each corner to a achieve a round shape that fits into your socket. This is trial and error. Rasp and fit, rasp and fit. etc.
Wipe some chalk inside the socket before each fit, and the transfer marks onto the blank will show the high spots to shave down.
When you have a tight fit, plane the handle's square corners to form an octagonal section that's a good fit for your hand..... round over the end.
When you've done a few, it will become second nature.
In use with a few knocks, the handle will distort and tighten to fit any irregularities inside the socket. If handles does work loose, a little oil on the end, then dip it into some sharp sand improves the bond..... an old chippie's trick
Good luck