How can I find a second supervisor?
Your last paragraph already says what you have to do first: Talk to your supervisor about potential second supervisors/mentors. You could bring a list of people who you would like to approach and ask about his opinion. Maybe he also has suggestions. Also make sure that he in principle agrees with the idea of a second supervisor.

Then you should be clear about what you want from a second supervisor: Regular discussions in person/email/phone? Feedback on drafts? Discussion or even joint development of ideas? Extended discussion via email? I somebody from a different place the supervisor of which I do not know would approach me about being a second supervisor, I would want to know answers to these questions and probably would have opinions about the answers, too.

The last paragraph also shows that you know at least one way to approach a potential second supervisor. Apart from personal meeting I would not recommend a non-solicited email from your side. If you can't meet the potential supervisor in person, I would suggest that your supervisor would make the first contact and let him/her introduce you.