I don't know where to buy one, but I made my own.
Start with a 1/4" steel plate. Then you'll need 2 drill bits. Let's say you want 3/8" fluted dowels. You'll need a 3/8" bit and another that is undersize by twice the desired depth of the flutes. If you want 1/32" deep flutes then you need a 5/16" bit.
Drill the plate all the way through with the smaller bit. Then drill in the same hole with the larger bit, but stop short of going all the way through.
Use needle files to enlarge the bottom of the hole to the larger diameter, or just a tad larger, leaving fingers to form the flutes.
Pound your dowel blanks in from the top side.
Depending on how much use the plate will get you may want to harden it.
HTH.
Art
I used to own a plate for making dowels that made fluted dowels instead
> of smooth dowels. I have not seen it offered anywhere and cannot
> remember where I purchased it before. Unlike the Lie-Nielsen Dowel
> Sizing Plate which makes smooth round dowels, the plate I am thinking of
> would cut flutes as the wood was pounded through the plate.
>
> Does anyone know where a plate for making various sized fluted dowels is
> offered for sale?
>
> John
>