Where can I find an online community specializing in the Z specification language, where I can ask specific questions about the ISO standard for Z?
There is the comp.specification.z newsgroup, available on Google Groups.
There are some mailing lists:
Additionally, there are mailing lists for tools that support the Z notation. For tools that support ISO Z (or a similar dialect), I doubt anyone would object to a question about the notation itself if the above sources are not useful. For example: