I make my toolholders from pieces of 1/2" square with a groove milled in one side. For round tool bits I just drill a hole to suit in one end of the square,deep enough to grip the bit and then slot the holder so that it can be clamped in the tool post. It,s simple and cheap,and you need only a few sizes to fit all your tool bits 1/8" 3/16" 1/4".
The square an have a 1/8"groove in one face,3/16" in another and 1/4" in the third side. Likewise another piece can be drilled 1/8" one end and 3/16" the other or 1/4",whichever suits. No need to go looking for specials.
Dont have a mill? No problem,a hacksaw and a file works just as well,just takes a bit longer.