Silver Pen Parts is where I get mine.
This shoild be the tap you are looking for to use the Baron section.
Tools Tap/Dies M8.4
This 8.5x1 may also work for making your own sections if you get the die as well. You could also use it for your finials.
I would also suggest while you are there, to order this one.
Tap/Dies M6.5
I believe this will fit the feed from the Baron, as well as the #5 nib sets from Meisternib.
For the cap to barrel threads, I would suggest single start. I'm currently using M12x1. I'm not sure where to get these, as I picked mine up here, used.
The single starts a much cheaper than the triple starts. I think they are also a little easier to use in the beginning. While I havn't used triples yet myself, I can imagine that they are, by their nature, a very agressive cutting tap.
Good luck with your venture.