YC's startupschool has co-founder matching. Advantage: people there actively look to find a co-founder.

Besides that, network. Go to startup events in your community. Go to <insert your favorite tech stack> meetups to find out if anyone has any needs. I think it will be very difficult because co-founders have to trust in each other and trust only comes over time with personal meetings and working together.

Regarding your pitch: What can you offer? Are you a tech person? Can you do marketing? Have you gained experience in other startups? How big is your network in their field?

There are the kind of things I would be interested if I were looking for co-founder.