As there are several options available below, we have this little guide (and recommendations) of the refills available:
The Pelikan 337 is a standard ball pen refill of excellent quality with a very large ink capacity.
The Parker QuinkFlow, Diplomat EasyFlow and Monteverde Refills are Hybrid inks of oil and gell and therefore are smoother than normal ball pen refills and start immediately. The German made Diplomat EasyFlow would be the Pure Pens Hybird Refill choice.
The Monteverde Gel, Parker Gel and Schneider refills are liquid ink as you would normally find in a rollerball and give a smoother writing experience with more vivid colour colours. They tend to not last as long as standard refills. The excellent quality, transparent (so you can see how much ink is left) German made Schneider refill would be the Pure Pens Gel Refill Choice.