With the Huntsman, you may be right. But remember, those BH209 groups with the NEF Huntsman were shot with the Hubbard plug. I also tried Blackhorn with the .25 ACP plug and primer carriers with mixed to poor results.
The wads are from Cabelas, I'm 99.9% sure that Sagebrush makes them for them also being from Sidney, NE. The ones I have are .518" diameter x .060" thick. They also make them .030" thick. If you had the gasket material and a punch, they could be made very easily. They were around $16 for 1000, or figure out to 1.6 cents a piece.
As far as conicals, Bullshop would be the least expensive, and they are top notch. You can have Dan size them to any full thousandth diameter between .499" and .505" IIRC. He also will size to some half thousandths sizes as well, the .5045" is very popular with the Bo Clerke barreled Whites.
I haven't been on marlinowners, I'll give it a look sometime.