One way would be to think of your game idea like a Cause that you believe in and want others to support. How would you promote and disseminate that to find fellow believers?

Talk about it and show what you're building. Those most engaged will be your short list to spend more time with and "date" to become a co-founder.

It's probably worth the effort to have a good idea around what the project needs in a cofounder too.

Have fun building!