Recommenders that are professors but not co-authors
People who cite your papers

A particular audience that is suitable for this are people who cite your papers. It is quite likely that the committee wants to see what impact (if any) your research has on your field, so they would want opinions from people outside your team that have read your research and find it useful. One way to find such people is to look at citations to your papers - they (obviously) have read your research, consider it valuable, and you should be able to find their contact information from their papers.