Where can I find official tournament and league rules for Dominion?
There are intentionally no official tournament rules for Dominion.
There is a long, meandering discussion of tournament rules for Dominion on BGG.
I was pretty happy with the tournament rules used for Dominion at the 2010 World Boardgaming Championship. Two things that it got right were
- An emphasis on game placement rather than points per game to determine advancers
- Playing two games per round and reversing seating order between games
But don't expect anything near consensus on the best tournament rules for Dominion.