As far as I know, all fountain pen kits use 'standard international' short cartridges or converters that are designed to just slip over the friction-fit nipple at the end of the pen section.
The better-known fountain pen retailers (eg, Goulet, Dromgooles, Fountain Pen Hospital) all offer converters. The things you want to look for, at least in my experience, include:
- screw mechanism - avoid converters that feature either a slide mechanism, or 'aeromatic' converters that have a rubber sac that you have to squeeze.
- metal body - most converters have a plastic barrel, but converters with a metal ring where the barrel meets the knob tend to be better than converters that are all plastic
- bead- resistant - ink flows better in a converter that has a provision inside the barrel to prevent ink from beading at the wrong end of the barrel when the pen is held in the writing orientation (nib down). Most commonly, this provision is in the form of a tiny, loose plastic or glass bead inside the barrel.
- disassembly - ideally, the metal ring is threaded such that the converter can be disassembled for cleaning and lubrication. However, this feature is relatively uncommon and probably carries a higher price if you can find it
Standard international converters typically retail for $5 or more when purchased individually. Some of the kit suppliers (eg, PSI) do offer converters at lower prices when purchased in batches, say a bag of five. I've used quite a few generic (unbranded) converters, and as long as they meet the criteria listed above, they have been fine.
Schmidt is a major manufacturer of high-quality pen components and offers several standard international converters; if you want to go with Schmidt, choose the K5 converter. There is also a K1 converter (all plastic) and a K6 converter that is designed for threaded nipples, a feature that is found only in a few name-brand pens.
Just be aware that most other name-brand converters are proprietary and only fit pens made by that manufacturer; they are also more expensive, sometimes as much as twice as expensive as standard international converters.
Finally, if you intend to offer converters, you may also want to have a small stock of silicone grease. Silicone grease is often used to lubricate the moving components of converters, especially those that can be disassembled.