How do you do it? …if you need in various sizes and different woodstocks.
Lathe?
Steel sheet with holes? Maybe @metaltechs can advice if its possible to fabricate that with the tools we have? I’ve seen pics of a metal sheet with holes of various sizes and you hammer your wood through the holes to give you the dowel.
Or buy them? Where from? Especially different wood stocks and sizes…aside from the bog standard packs available in B&Q.