I think the choice for your co-founder(s) depend on yourself and your own skills as you will need somebody who complements your skills. This relation is about trust so either you should pick somebody you can rely on or trust your life on or you search for a person and establish a deep relation via extensive bonding (e.g. tandem skydiving, something that requires high levels of trust). Another important aspect is the personal network and the mindset.

In conclusion:

1. Know yourself and you will see realize what you lack
2. Build the relationship to your potential co-founder around trust
3. Personal network
4. Mindset (Unbreakable attitude, positive, go-getter etc.)