Here is a prefect place.
If you read French, another nice place is:
http://www.strataegis.com/forums/vie...f95e57bb0076c4
There is also the BattleField and GoneGold, but they have been quite dead lately, so I can't be bothered to find the URLs.
There is also a dominions II IRC channel... you can find information about it elsewhere in this forum. I do not use it, so I cannot tell you how many games are organized there, but I am told that one can learn an awful lot from the discussions there.