The resources listed here are provided by people and organizations I trust, though I cannot vouch for all of them personally.
AORTA Training in anti-oppressive facilitation
AORTA (anti-oppressive resource and training alliance) is a cooperative organization that provides organizations with training in anti-oppressive facilitation and structural change. They provide some virtual training content at various levels of experience (beginner/foundations through apprenticeships).
Workshops with Just Practice Collaborative
Just Practice Collaborative is a cohort of some of the most experienced and well regarded Transformative Justice practitioners in the U.S. (Shira Hassan, Mariame Kaba, and more). I received my training on Community Accountability through them, at an in-person training in NYC. They sometimes provide workshops open to the public (I will warn you though that sometimes the website is just blank), both virtual and in-person. You can also hire them to facilitate a workshop for your organization. They also made a mixtape of training content which you can self-pace, called Steps to End Prison and Policing: A Mixtape on Transformative Justice.
Training in Circle Process with Kay Pranis
Kay Pranis facilitated my training in Circle Process, a Restorative Practice she was trained in by Indigenous Plains communities. Kay offers this training several times throughout the year at different venues, most predictably at Eastern Mennonite University in Virginia. You can also google her or google "Circle Process Training" to find opportunities to learn this practice. Kay is also the author of the Little Book of Circle Processes and the Circle Keeper's Handbook, which are both great guides if you've already received the training. I highly recommend attending a training before trying to facilitate based just on the books.
Amplify RJ is based in Southern CA and provides learning experiences on Restorative Justice for educators and beyond. They have online/virtual trainings frequently; I highly recommend following them on instagram for announcements about their awesome programming and podcast. Amplify RJ was founded by a really cool guy named David.
Holding Change: The Way of Emergent Strategy Facilitation and Mediation
A book by experienced transformative justice practitioner adrienne maree brown, with writings by other talented, experienced facilitators.
Post-COVID-19 there are more and more opportunities to be trained in mediation virtually. Choosing the mediation training that best suits you involves having a solid understanding of your own lens/perspective/worldview, because there are many types of mediation and many companies and universities that offer mediation training, all of them varying in their level of awareness about power dynamics, injustice, structural violence, and anti-oppression. When looking for a quality training, I would look for (a) who are the facilitators and what is their experience/does it align with the experiences I need/want, (b) what are the organization's claimed values and mission, does that align with what I need/want, (c) what kinds of mediation are they saying they will prepare me for, does that align with what I need/want? You can often reach out directly to a coordinator or facilitator and ask--"I'm interested in this training in order to be able to do X, Y, Z, will this training be a good fit for me?" I've included some links where you can look for courses, though I can't vouch for the trainers or courses themselves. Good luck!
This training video by Community Mediation Maryland is a great example of a facilitative mediation approach, with two co-mediators. I'd recommend taking a live training so that you can get some interactive practice, but if you want an overview or a refresh of some basic concepts, this is a decent resource. There are four main approaches to mediation: facilitative (neutral process facilitator reflects back participant language through filters), transformative (facilitators focuses on learning conflict skills and understanding), inclusive (no filtering, participants self-determine and express), and evaluative (facilitator assesses party's positions and makes recommendations). The training you decide to take will be framed around one of these approaches, so you might want to ask coordinators which approach before you sign up and pay.