Founders Nation - Find or become a startup cofounder
Foundler - Online hackathon weekends. Attend every other week.
CoFoundersLab - Meet 250,000+ entrepreneurs online
CollabFinder - Find cofounders and makers to help build your project
Ship That Shit - Join 3 no BS events to help you ship that shit
Co-founder Question Cards - Questions to ask your co-founder to know each other better.