A lot of the advice around finding a co-founder is focused around college/childhood friends. And I think that's why most brush off online forms of finding partners to work with as inferior :shrug: - I think this is a mistake.
Quit viewing it as finding a co-founder and just view it as finding people to build and ship cool stuff with. It takes some of the pressure off the interaction and gives you all a chance to just practice working together.
Also if you want to collaborate on something find my email on my profile and here is my github https://github.com/roymoran. There just did it. If it works out cool, if not cool.
People that just want to collaborate and build stuff will do so. I don't think you need much convincing.
The other option is to solve a problem and see who is obsessed with solving the same problem.