Reddit mentions of Pelikan Ink Converter for Standard Cartridges (999128)

Not to slight Himiko, but I think the converter is overwhelmingly the way to go over refilling cartridges (something required for some pens but inherently messy, they wear out, and it's not a great experience A for your first fountain pen). I have this one that works perfectly in my Muji. I have heard that most international converters leak in this pen, so I'd be careful to get either this one, or I guess the Schmidt that Himiko mentions above, although I don't have the link for it. I know that everyone has preferences on something so subjective, but I'll also suggest that you buy yourself a bottle of Diamine or Iroshizuku ink first time around, you'll be obsessed in no time. I mean, there's a ton of wonderful ink out there, but both of these brands are so well behaved on all paper, and and have very few duds. Make sure it says 'fountain pen' on the front, as dip-pen ink will ruin your pen. Oh, and welcome to the world of Fountain Pens!

This one fits and works just swell :)

yes it is, I was actually cleaning it up to try out the pelikan converter, which I've just put in right now. So far so good, but I'll report back tomorrow in case of leaks!

Depends on which model you get. I have the Starwalker Extreme (in black) and am using a Pelikan Converter Link.
While investigating which the issue, I heard about Pelikan, Schmidt K5 and Faber Castell converters working.

I’d recommend going to a nearby shop that sells one of these converters and seeing if it fits.