https://lemmyverse.net/communities This is a good start in search for communities. You can enter your home instance(in your case lemmy.ca) by clicking on the house symbol at the top right and then search for communities like “security”.

When you click on one, it’ll automatically lead you to the adress based on your home instance and you can immediately subscribe to them form there.
https://lemmy.ca/c/cybersecurity@sh.itjust.works could be of interest for example