Spare / unit of nib, upper+lower reservoir

G or Hunt 101 nib + a feeder w/stem and upper reservoir
Quill nib + upper reservoir feeder
Includes other small supporting pieces.

This may require a little tinkering depending on the nibs.
Start using it as a dip pen. Once the feeder starts holding ink, you can switch to using it as a fountain pen.
After all, replacing nibs is easy.

Alternatively, you can purchase a matching nib section (this nib unit in the nib section) at the time of ordering the pen.