If you are looking out for senior leadership level professionals then it won't happen that they are not there on LinkedIn if not on any job portals. But if it happens that they are not socially active then you can look out for the following options:-
1. Employee referral is one option you can try.
2.But if the company is a brand and the management is willing to splurge some extra money then advertisements in best-selling newspapers could be another option.
3.Forums are also other way of communicating with senior leadership the requirements of certain posts in the company.
I hope this answer will help you and you hire better candidates.