There is no central entity governing the BPS rules, but the documents published for EUDCs and WUDCs have a strong influence and are usually very well-written.
There is no central source for finding out about international competitions. Most competitions are announced on Facebook, so it helps to join a few debating-related groups and mark upcoming competitions as "interested," which will usually lead to Facebook suggesting other upcoming tournaments.
The largest international competitions are the World University Debating Championships (WUDC), and, in Europe, the European University Debating Championship (EUDC). Both are generally open for anyone to attend, but require registration a considerable time in advance.
Most competitions are organised by invidivual clubs and announced in the Achte Minute and the VDCH-Verteiler. More centrally organised are the regional championships as well as the yearly german-speaking debating championships.