I've bought springs and clicker mechanisms for ballpoints from Richard Greenwald and from Milans Pen Parts (they also have springs for roller balls, which are different.)
Fountain pen nib units from MeisterNibs.com (JoWo), fpnibs.com (JoWo and Bock), beaufortink.co.uk (Bock), Goulet Pens (JoWo).
Choose either Bock or JoWo and stick with it for a while, because the tap needed to cut the threads to hold these are not the same, and while they aren't as expensive as the gold you are planning to use in your pens there's no point buying both sizes if you don't have to.
Fountain pen ink "cartridge converters" can be found all over the place - Richard Greenwald, Exotic Blanks, Amazon, your local fountain pen store.
Roller ball refills I get from Exotic Blanks.
Ballpoint refills from Exotic Blanks (the Schmidt refills are very smooth writing) or from Staples (when I want to use brand names like Parker or Cross.)