Great question. The short answer is that a campaign manager can be anyone you think is right for the job. There are some policy firms that can provide experienced campaign managers. But this is not necessarily standard or accessible to every candidate. You are hiring a contractor to fulfill roles and responsibilities that you set forth for them.
Certain professional backgrounds may lend themselves to being a better campaign manager than others. For example, it may help to seek folks with degrees in law, public health, social policy, social work, communications, or history. As a social worker, at least 1/3 of my education was spent learning how to understand and organize around policy to create social change. There are specialist social work programs specifically for law and social policy (MLSP and PhD) or community organizing. A social worker is a great option for a campaign manager - as are other types of professionals.
However, having a degree only has to be a requirement if you make it one. Some people experienced in working for political campaigns may do so out of personal interest and intrinsic motivation. They may not need a degree to be effective. Regardless, anyone you hire should have a knowledge of and interest in politics. They should also believe in your platform and your ability to bring your policies into fruition.
You can find someone interested in serving in this role by making it known the position is available. Contact professional associations like the NASW. Post the position on a hiring board. As the local organization affiliated with your party for a recommendation. Contact local colleges for soon-to-be-graduates looking for a job.
Running for public office is a learning experience forged by trial and error. Choosing and hiring a campaign manager is no exception. Best of luck on your journey in politics.