I have found my last few freelancing gigs by posting my availability in Slack groups. As an Elixir dev I posted in an Elixir-specific Slack group. I also post in the HN monthly threads but I haven't gotten many leads from those.
It also helps if you have an established profile, e.g. an active Github profile with open source contributions.
It also helps if you have an established profile, e.g. an active Github profile with open source contributions.