Z88 on the net.
Rakewell is the best source for z88 hardware and software today. If you need something, just drop them a mail. You also find various documentation for the z88.
SellMyRetro. You maybe able to pick up some z88 hardware and software here.
http://sellmyretro.com/category/Retro+Computers
Cambridge z88 wiki. Everything you need is here.
https://cambridgez88.jira.com/
Facebook. There is an active z88 community group on Facebook. Go check it out.