From Discworld MUD Wiki
An ink well is a vessel and a baggage.
It has a capacity of 250 drops.
It is made of tin and has dimensions of one inch by one inch.
It looks like: This ink well is a small tin bottle with a wide mouth and a close-fitting cork.
It has a weight of 1/9 of a lb when empty.
They can be purchased (with Inks in them) at various locations.