Last week, Scrib talked about the Kaweco fountain pen. Nero also sells some Lamy pens, for those that wanted to try something else. But once any ink they came with has run out, what can you use to refill them?
Personally, I tend to use bottled ink rather than cartridges, on the grounds that there's less plastic involved (especially if the ink comes in a glass bottle!), but that's just my preference.
If you've treated yourself to a Kaweco, Nero stocks not only packs of cartridges in 11 different colours, but also has 30 ml bottles of ink in five of the colours in the cartridges. Get yourself a cartridge refilling kit (cheap as chips; search online), and you can rinse out the cartridges and refill with bottled ink. The pens will also take an international standard short cartridge, rather than require a proprietary format.
If you've got a Lamy, Nero has you covered. There are converters so you can use bottled ink, there are cartridges in seven different colours, and there are four different colours of bottled ink. Unlike the Kaweco, an international standard cartridge won't fit the Lamy, but once you have either an empty Lamy one, or the converter, you can refill with any ink.
Not only that, but Nero stocks a selection of Diamine inks in 80ml bottles (these appear to last forever!), so, armed with a converter/refilling kit, the world is your oyster.
If you've yet to disappear down the rabbit-hole of fountain-pen inks, let me invite you in!