A Google of Oil Hole Covers took me straight to The Nipple Shop
But it is actually what you're after and they're based in Bolton.
It never occured to me that these things would still be available so it's very good to know. They may have minimum orders though but as they are in the UK may be worth a call. If you had to buy a few I'm sure there's a market for them on here. Looks like they are all nickel plated though which is a shame.
Check that you don't need a grease type nipple. I have a 1936 Hobbs and that's what's on there. I think it's still for oil but it looks like it's for a grease type gun.
1936 Hobbs of Barbican, 1954 Viking "Ian Steel" model, 1984ish Raleigh Royal, 1991 Orange Prestige.