@j12t Are you looking for IdPs or RPs? Here are some test RPs:

webmention.io/

iam.sebadob.dev/auth/v1/fedcm

Not sure about the status of lastlogin.net/ - maybe @apitman can comment?

Or you could try my RP at login.mythik.co.uk/ - if this works you might even be able to get into discourse.mythik.co.uk/ with it.