If you want to make fountain pens you'll need to get specialty taps for the feeds. They are proprietary threads sizes unique to the nib maker so not a stock size. The nib sellers will usually have the taps needed.
For the barrel/cap and section/barrel threads you can start off with single start threads to see if you like making pens from scratch. If you do then you can graduate to the double start or triple start threads which are specially made and expensive because of the low demand.
There are various places to get the single start threads but one with a large selection of inexpensive taps is
Victor Machinery Exchange. Get the drill bits at the same time. Makers usually gravitate towards the metric threads because they are easy to work with.
There aren't too many suppliers of multi-start taps and dies so Turners Warehouse mentioned above is one source and the other is
Beaufort Ink. They also sell Bock nibs and feeds along with the taps for them. Note that at the moment they are moving over the next couple weeks so not shipping orders until the move is completed.
Classic Nib also sells some Bock nib/feeds and the taps. Disclaimer time. My wife Marla makes pen blanks for these companies.
Sometimes here and on Facebook people run group buys to get the taps made for a better price but you might have to wait some time before another occurs.
Your last alternative is to get a metal lathe and cut your own threads, bypassing the need for the taps and dies.