You're gonna want to use Stubhub...or the lottery line at 8:30 the morning
before the game.
Avoid flashseats...their fee is $100 as opposed to Stubhub's $50. The equivalent seats at stubhub end up being slightly cheaper. My dad and I just bought tickets for all three games...it took about 3 hours of comparing on all sites. We are both very cheap.
A tip:
Buy your game 3 tickets NOW. We are right behind home plate for the same price that we are behind the dugouts in game 1 and game 2. You get your money back if game 3 doesn't happen.