This is a great question! I can speak from my experience co-founding Tachyus, and experience joining very early stage startups and observing founder dynamics.
I think the first question is, why are you looking for a cofounder versus hiring a first employee? Both have pros and cons. Cofounders typically receive at least an order of magnitude more equity compensation then employees. This aligns them much much more tightly with the company, which can be great if your values, vision, ambition, motivation, energy, etc. match, and can be a disaster if they don't. I would consider the decision as carefully you would a proposal for marriage (no joke!). Given the importance and impact of this decision, I would only consider people that you know very well as cofounders. Pick from your inner circle of friends or colleagues. Ideally it's someone you have worked with before. If you know them outside of work, work with them for several weeks if possible before cementing a co-founder relationship. I would advise against trying to find a co-founder that you have never met before, and instead consider making someone new that you're very excited about a well compensated (equity wise) first employee.
Happy to jump on a call if you'd like to chat more about it!