Fenway sells out. I used a ticket broker for my seat. There may be people selling tickets outside of the park. There may be a few tickets for sale at the walk-up window.
I wouldn't fool around with anything other than tickets in hand before heading to BOS.
Words to the wise: DO NOT bring your car. Just forget that idea. I squeezed a game in because was in the neighborhood--if PVD is in the neighborhood. I didn't have lots of time to figure it out so I parked about 1/2 mile away in a hospital's parking garage. It was expensive, but beat all the other options. Had I more time, I would've parked at a Park n' Ride T stop and taken the T in.