Is there any reason you are trying to use SAML? We have plugins for OpenID Connect and OAuth2, which are generally a lot easier to configure.
We don’t have any officially tested IDPs. However, the top google result for “open source idp” is Keycloak. If you search here on Meta it looks like other people have managed to get it working with our openid-connect plugin (e.g. here). Maybe try experimenting with that as a starting point?