The refilling of fountain pens can be done through different ways. In the USA alone, there were patents issued for Button, Click, Lever, Matchstick and coin types of refill. All of the above methods were applied to get to inject a little pressure and force or deflate the plate inside the pen to allow ink to flow through.
The best pens may need eyedropper fillers. There are also some self-filling pens which have a sac in them. It works by putting pressure on the sac and the pen automatically gets filled or emptied of ink.
The most recent method introduced is the use of cartridges or rather the international cartridges which are referred to that way because of the large volumes the European companies are able to use in the fountain pens.
The use of piston filler is another popular method. This method makes use of piston in the pens, which is driven up or down in a barrel of the pen by twisting a knob at the end of the pen. Nowadays most pens just use piston fillers or cartridges. This is because there are converters that have been introduced to convert the cartridge refill pen into a piston one.
The eyedropper type of filler ink pens are the ones with a maximum load of ink, where as the one with the least is the piston type. This is because the piston manages to occupy half of the space of the pen's barrel and leaving very little room for the ink. The only disadvantage of the eyedropper is that it tends to be a bit messy.
The international cartridge is mainly used in all sorts of pens all over the world, but some particular pens will only accept the cartridges of their kind. Then there are some pens which can not accommodate or take a converter. It then requires these pens to be filled using a using a syringe.
Of course getting the ink to use is not a problem as there are many types to choose from. Ink bottles come in all shapes and designs. The fountain pen users also like to keep some refills ready just in case they run out of ink.
Some particular users also love to refill their own cartridges by themselves even if it means messing around with ink.