Lots of zerk sizes out there. It could be SAE, say 1/4"-27, or metric (M6, M8).
Although I have one of those zerk collections that you can get at HF, it does not have
all the common sizes. I just bought a bunch of M8-1.00 since the kit has M8, but
only in 1.5 mm thread.
In your case I would remove the broken-off zerk remnant and match it up with a bolt
of known size. Then pick up some zerks at your local auto supply.
Note that the threads are tapered, like pipe thread, so a bolt will not easily thread into the
hole.