I'm not sure of the size but I had a couple on my front loader that wouldn't take grease. Turned out to be the holes inside the pins to be plugged up not the grease Zerks. I just removed the pin and it was easy to clean out. I reinstalled it and it's good to go.
I've had to replace zerks on my FEL & grapple. I take a working zerk from the opposite side of the FEL or grapple and trot on down to my NAPA dealer. Makes things a whole lot easier. Surprisingly, as I remember, the FEL had some European(??) sized zerks and the grapple was SAE. I got the SAE zerks from NAPA and the European ones from the Kubota dealer.
You could go to Amazon and just buy an assortment package of zerks. You get around 100 of various sizes and configurations in each of the assortments (SAE and Metric). Not terribly expensive, and handy to keep around.