I have a hand tapper, the kind that costs $79.95 from Enco. Like many import "sets" these days, these handy machines don't come with all the sizes. They come with (I think) Nos. 6, 8, 10 and 1/4, 5/16, 3/8, 7/16, 1/2, 5/8 tap holders. The first holes I needed it for (I'm not making this up) were No. 12 threads, the second ones were 9/16" threads. Exactly the ones they didn't make. Aarghh.
These tap holders are fairly complex, also they are hardened and ground and made to close tolerances. I know this is a super-long shot, but does anyone know of anybody who sells the No. 12 and the 9/16" tap holders? Failing that, I'm wondering if it's within my limited abilities to make them. They are basically a round body with an axial square hole, which has to be closely centered. Both the body and the central hole have to be within a thou or two. There is a tang on the top which is drilled and tapped for a set screw which retains the tap, and there is a semi-spherical recess for the retaining ball which holds it when you snap it into the spindle.