It is an ER16. They are commonly available. Also due to the thread of the collet nut you can use the products A2ZCNC makes.
Trying to find the original Dyna tooling is almost impossible. I do have a few that I am keeping. It is the clover like nut that would be the most difficult to find. It is a very interesting design that would seem to be viable for tool length repeatability. Yet nobody else seems to have adopted it.
Tool Holders Remember you have an ER16 nut thread (22 x 1.5mm), not the 3/4-16 of the Sherline spindle. Purchase accordingly.
Sherline of course would also be a great source. They will offer both also.
Personally I would expect more accuracy and less run out from a tool holder that used the conically tapered bore over the spindle noses external thread.
That would really depend on the accuracy needed. Both companies offer good tooling for these sized machines.