@j12t Are you looking for IdPs or RPs? Here are some test RPs:
https://iam.sebadob.dev/auth/v1/fedcm
Not sure about the status of https://lastlogin.net/ - maybe @apitman can comment?
Or you could try my RP at https://login.mythik.co.uk/ - if this works you might even be able to get into https://discourse.mythik.co.uk/ with it.