Looking for an online community specializing in the Z specification language, where I can ask questions
There is the comp.specification.z newsgroup, available on Google Groups.
There are some mailing lists:

ZFORUM mailing list
Zed Freelists mailing list

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:

Community Z Tools (CZT) mailing lists
ProofPower mailing list