There are probably too many ways to respond to this question without making a full post, but a few examples:
1 - Reach out to other freelancers - many freelancers actually have too much work and will refer some leads along to others they trust.
2 - Recruiters - find an agency recruiter who you can trust and tell them to keep an eye out for you. Recruiters love placing contractors, as it is basically an annuity revenue stream.
3 - General outreach - just reach out to people. Founders, meetup leaders, CTOs. Do a little research and send short messages declaring your availability and interest.
4 - Let the work find you - make sure you have a discoverable presence on LinkedIn and elsewhere.