As far as I know, all fountain pen kits use the 'international standard short cartridge'. These are available from most pen kit suppliers, and there are a number of vendors who sell them over the internet. Some of the more reputable dealers include HisNibs and ISellPens; Goulet Pens has an excellent reputation and an extremely broad product offering.
I know that some vendors claim that refills can be purchased at 'any stationary shop', but in North America, that is really not true - outside a few very large cities, 'brick and mortar' stationary stores rarely carry an extensive line of fountain pens and accessories, and what they do carry tends to be refills for specific brands (MontBlanc, Cross, and perhaps Parker). Stationary stores in larger cities do have a broader selection, but they will still be strongly biased toward name brand pens and related accessories. And most of the more famous 'brand name' pens required proprietary cartridges or converters rather than the 'international standard short cartridge'.
I'm a fountain pen user, and I have a pretty broad collection of pens. I my opinion, cartridges are pretty spendy - its much less expensive to buy bottled ink and then use converters. However, cartridges are more convenient if you are traveling a lot. You can also refill cartridges using a blunt syringe.