Grease nipples are made with a variety of threads, depending where and when the bits to fit them to were made, but the most common ones are BSP or metric.
These are a bit pricey but at least you will have enough to last a lifetime
I bought some about a year ago, I only needed half a dozen. At one time you could go to your local motor factor and get them, I tried them all around my place, even Halfords, but it seems they run out years ago as most things now are sealed for life, but your local engineering supplier or farming supplies should be able to help you out, but most do have a minimum order. I finished up buying metric ones.
I hope this puts you on the right trail.
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