How do I find a management job after being self-employed for 20 years?

You write on your website that it is more important for a company 'to interview the individuals for which the candidate worked' more so than the candidate interview. After 20 years (or in my case 11) I have no idea where to find my supervisors even if I could remember some or their names. I've offered vendor references who I have done business with over those 11 years, to show an honest business person and likeable if nothing else.
And while you may have hundreds of thousands of applicants who haven't had trouble from being removed from corporate for a number of years, I was just interviewed by a search firm such as yours and was told their clients do indeed look for someone who can give a 2 weeks notice from their district manager position and move laterally right into their organization. Hopefully, you're right.

Maudie, you make a good point about self-employed folks having a more difficult time finding references. Lack of appropriate references is one of the few legitimate reasons for holding back on a candidate (regardless of whether it results from self-employment or from some other issue).

If you can't find former supervisors, then find the next best thing, which is what you appear to be doing. Good luck to you.