The Timeline of the Theban Tribunal makes reference to the New Slavonic Tribunal... is this Novgorod?
Yes.
Don't have the books on hand, but I believe most of the important references are in Guardians of the Forest.
Novgorod was not one of the 'original' Regional Tribunals set up in 773. The New Slavonic Tribunal emerged shortly after the Schism War as a break-away from covenants in the eastern part of the Rhine/Transylvania. Eventually it became fully recognized and was renamed (by 1220 though I don't remember when exactly it happened) the Novgorod Tribunal.