How can I find a second supervisor? I'm in the second half of my Phd in a technical field in Germany. As it is common in Germany, it is not a structured PhD program but an independent PhD without any course work/mentors etc. I am not very satisfied with my supervisor because he cannot help me at all, since my topic is not his primary interest and he is very busy since he is not yet a professor. I do everything alone from choosing the specialisation of my topic to writing (my first) papers, and I got used to that.
I like my topic and I get the money and besides the bad supervision, my supervisor so I decided not to change the PhD. However, I would really like to find a second supervisor who could give me advise and has a better reputation in my field. 

How should I approach potential professors with my request? I would not need too much help or any money, just the name of someone who is at least a little known in this field and I would be willing to do some work for them. Is it very uncommon and unlikely to find a professor who is willing to do that?

I tried once at a summer school to ask another professor and it would have worked out but I experienced afterwards that my supervisor was not at all happy about that so I have to make sure beforehand that he agrees with my choice.
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.