Just for the record, calligraphy pens can be fountain pens too, so best to call these dip pens, meaning they need to be dipped in ink, rather than having an integral ink supply. Dip pens were made both for standard writing and fancy writing (calligraphy).
Watch out for the folks who call dip pen nibs "fountain pen nibs", like this one:
Vintage Lot of 26 Fountain Pen Nibs and Esterbrook Box | eBay
Esterbrook did make both dip pens (what we would call a nib, was actually the pen, and the long skinny thing that most would call the pen is called a pen holder), and fountain pen nibs. The fountain pen nibs were sold as screw in units, with feed attached. Unfortunately, they are not a standard thread, so to use them in a home made pen, you would need a metal lathe to tap the threads for them.
Here is what the Esterbrook nibs look like (in their individual boxes).
LARGE LOT OF VINTAGE FOUNTAIN PEN NIBS: ESTERBROOK, 14K, PARKER 45 | eBay
Dan