If your pen uses International cartridges then you may be interested in buying them from Morrisons. They are priced at 25 for 10p, one colour only which is a great blue. I have been trying to work out the maker, possibly Waterman, it is an excellent quality.
They are sold out for on line shoppers, as you can imagine at this price, you can often pay £5 for 20, it might be worth going to Morrisons.
In pursuit of a first-rate modern flex pen, I have acquired two LE edition Italian flex pens. I decided to go for some brighter colours as well. The top pen is a piston-filler Aurora Optima with 14k "F" flex nib, and the lower is a converter Scribo Piuma "Levante", also with the Scribo "F" 14k flex nib. Of the two, the Aurora is a less-flexible nib, although it is new and may loosen up with more use. The Scribo nib is, as usual with their flex nibs, wet and superb. The inks are Pilot inks.
I spend a lot of time daily with Gripmaster hand exercisers in order to get some strength back in my somewhat impaired writing hand and fingers, and must therefore apologise for my degraded handwriting; I hope a temporary thing.