When I am planing my new trip I usually make a youtube search on that country or area to get an idea of the tango danced there. If I like a venue or festival I try to contact the account owner via youtube. Some times it don't work sometimes it works relly well and I have had a friend waiting for my visit!
This site has been a helpful start. It gives you the organizers email so I have been able to check if the event is still valid. You find the map and other information as well.
New York - http://www.newyorktango.com/