Consider looking at medical school alumni associations. You can look for docs who graduated in the 1950's who are likely headed into retirement. You may be able to sponsor a reunion event or provide a contribution for mailing which could include your brand. Also, working with a professional society such which may have demographic information on its members it may be willing to sell is an option. For instance, in the US the American College of Radiologists is an organization 90% of radiologists belong to. Similar organizations exist for other professionals internationally.