[mooreds]

If you are interested in a particular technology, googling for "slack for <technology>" usually works.

If more interested on cross technical concerns (leadership, regional) I have found the best thing is word of mouth (I know several Colorado slacks, but found them through people I met at meetups). Though again you could try Google.

I have also found that most non project oriented email lists I am on have a link to their slack in the email footer.