There are several ways to find a mediator.
Many mediators get work through word of mouth. Ask your friends, relatives and business contacts if they have had a good mediation experience and could recommend a mediator for your problem.
If you are using a lawyer, then they may be able to recommend a mediator to you with whom they have had a productive session.
In some places, like Hong Kong, mediators may also be accredited by an organisation (e.g. HKIAC, HKMAAL). In Hong Kong, mediators may be either on a general panel (i.e. accredited to mediate any type of dispute other than family disputes) or on a family panel (i.e. accredited to mediate only family disputes).
You may want to use a mediator who is experienced in a particular type of dispute. For example, someone who is an architect or engineer may be a good choice for construction disputes as they will understand the context.