Where's the best place to meet my future co-founder/s online?

Is there a dedicated platform that allows one to explain what they're building and that they are specifically looking for co-founders? I'm currently enrolled in StartUp School from YC, but I'm looking for other specific options/services. I'de love to know them if you can share some products/communities here. For example, I'm building product A, and I'm looking for help in marketing. I'm willing to give n% amount of the company.