AFIK, all kit fountain pens use the 'standard short international cartridge". Converters are widely available.
While Amazon is one obvious source (eg,
https://www.amazon.com/SCHMIDT-Foun...=fountain+pen+converter&qid=1623281211&sr=8-5), I suggest looking at a specialist fountain pen supplier such as Goulet pens who can provide the converter, and also offer an extremely broad selection of bottled ink.
Converters are not expensive or sophisticated, but quality can still vary quite a bit. I've used inexpensive Chinese converters and they work OK. In general, a name brand such as Schmidt is going to be a bit more expensive, but it is less likely to have problems. I suggest staying away from the inexpensive all plastic models. PSI offers two versions - the more expensive version is fine and works well, but I've found that their less expensive converters tend to be a bit leaky.
A point that should be emphasized is that you need a 'standard international converter'. Schmidt is a well-respected German manufacturer of pen components who happen to make standard international converters. However, not all 'standard international converters' have a prominent brand name. In fact, many converters with a prominent brand name are actually proprietary and fit only pens made by the same company. Looking for a converter at Amazon can be confusing because they randomly mingle standard international and proprietary brands. This is another reason to opt for a supplier like Goulet - fountain pens are their only business, and they understand the market and the options very well and can help you select a converter that will fit your pen. That said, there are a few other well respected fountain pen companies - Vanness, Dromgooles, Goldspot, Anderson Pens, nibs.com, etc.
