So you’re trying to do Scrum well because you heard it gave you great results. You know that the ScrumMaster role is critical. How do you find the right people to fill that role? Here is a list of several roles that people commonly leave to become ScrumMasters, and a few not-so-common roles as well, all ranked by how well those people typically do once they become ScrumMasters. From Worst to Best:

Worst: PMI-trained Project Managers (PMPs). Too focused on control and cost and schedule. Not easily able to give teams the space to self-organize. Not able to detach themselves from results and focus on the process, values and teamwork needed to make Scrum successful.
Bad, but not awful: Functional Managers. The power dynamic can be a serious hindrance to allowing teams to self-organize. But, good functional managers already are good at building teams, and empowering individuals to solve problems.
Bad, but not awful: Technical Leads. Here, the biggest problem is the desire to solve all the team’s technical problems instead of letting them do it. Now, instead of detachment from results (project managers), it’s detachment from solutions.
So-so: Quality Assurance people. Good at rooting out root-cause for problems… just need to shift from technical mindset or business mindset to cultural and process mindset. Another problem is that QA is not nearly as respected as it should be and QA people typically don’t have a lot of organizational influence.
So-so: Junior techies: Enthusiasm can make up for a lot of gaps and naiveté can help solve problems in creative ways, but there will be a huge uphill battle in terms of respect and influence in an organization.
Good: non-PMI-trained Project Managers: rely on teamwork and influence rather than tools, processes and control (of course there are exceptions).
Awesome: Executive Assistants. Respected and respectful, use influence for everything, know everyone, know all the processes, know all the ways to “just get it done”. Of course, they don’t usually know much about technology, but that often turns out to be good: they don’t make assumptions, and are humble about helping the technical team!

The ScrumMaster creates high performance teams using the Scrum process and values. The ScrumMaster is not accountable for business results, nor project success, nor technical solutions, nor even audit process compliance. The ScrumMaster is responsible for removing obstacles to a team’s performance of their work. The ScrumMaster is an organizational change agent.

Other things you might want to consider when looking for a ScrumMaster:

Does the person have experience managing volunteer groups?
Does the person have good people skills in general?
Does the person want to create high-performance teams?
Can the person learn anything – business, process, technical, people, etc.?

Bottom line: try and avoid having PMI-trained project managers become ScrumMasters. Even with good training, even with time to adjust, I often find that Scrum teams with PMI-trained project managers are always struggling and almost never become true teams.