If you have vintage inkwells, usually they are missing their inserts (a/k/a ink pots or liners). Here are two sources for the inserts, both of which are in the UK:
Porcelain: https://www.ebay.com/itm/112396427327
Glass: https://facetsglass.co.uk (or the same via eBay: https://www.ebay.com/itm/362966765040)
They explain how to measure for the correct size.
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