What do you buy for your pens that fills it with ink?

Answer 1

probably something called 'refill'.

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A ball point pen will normally have a "refil". An ink Pen with nib will either have a cartridge which fits in side and passes ink to the nib or will have a rubber bulb which sucks up ink from a bottle and is fed to the nib by gravity
You will buy the pen's refill. But some pens will not have refills, so you can't buy it.

Answer 2

an ink well

or ink cartridges

or if a ball point - biro insert