Sadly, many manufacturers don't supply converters with their pens as a cost-saving measure. We make up for this by either bundling a converter free of charge with pens we supply or else offering a discount when you buy a converter with a pen. Some brands of fountain pen need a brand-specific converter (more information here).
If your pen was not supplied with a converter, it may just be that the barrel is too short to take one. Examples of such pens are the Kaweco Liliput, Sailor Sapporo Mini, Yard-O-Led Pocket Viceroy and Conway Stewart Dinkie.
If your pen takes standard size cartridges, then a standard size converter such as a Schmidt K5 (as in photo below), Schmidt K2, Pelikan, Faber Castell , or Waterman/rotring should fit (this last one is a bit slimmer than the others). Check the barrel inside length and diameter before ordering. Not all converters are created equal.
If your pen does not take standard cartridges then it's most likely that you'll need the manufacturer's proprietary converter. Browse this page to see if your pen brand is listed.
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