Bottled ink is easy to find when fountain pens are popular, but harder to find when they are less popular. Their popularity seems to ascend every few years and then fade after a few more years. If you live near a store that specializes in fountain pens, there is no problem. Otherwise, you can order bottled ink from several suppliers on the Internet. Do not use calligraphy ink in a fountain pen.
Fountain pens have come full circle. A very early fountain pen used a glass cartridge fitted onto a rubber cone behind the nib. The user filled the glass cartridge and pushed it onto the rubber cone. Later rubber bladders and piston systems came along. Some of these got to be quite elaborate mechanically, while others were quite simple. Almost fifty years ago the plastic cartridge became popular and pens now use cartridges.
Step 1: Suitable Implements for Refilling
You can also get a plastic syringe with no sharp needle at a pharmacy. This have a curved end and are for washing a baby's mouth with a solution when the baby is teething.
You can also use a glass eyedropper. These are also available in a pharmacy.
All but the hypodermic needle require some modification. See the next step.