you have two options for fenway tickets (unless you're willing to spend the grand, monster tickets are right out). oh, and pretty much (if not all) every red sox home game is sold out already. that's what you get with a small stadium and a popular team. but, you have two options:
1) the ballclub reserves 200 tickets for day-of-game walkup sales. to get them, you have to line up outside the ticket office 2-3 hours before game time. they also sell blocks of seats that have been returned or not used for whatever reason (say, the away team doesn't give away all their allocation). means you have to line up ahead of time and is pretty hit or miss. but you do get them at face value, no surcharges.
2) spend $50 and get a red sox replay membership (you can find it on redsox.com if you snoop around the buy ticket link/page). here, season ticket holders who aren't going to make a particular game tender their tickets back to the franchise. the franchise sells them for pretty much face value, 10% or so markup. one thing i've noticed, though, is that there will hardly be any tickets for sale for longer than 30 seconds before they're purchased (by ticket resellers, i'm guessing). but the day before and moreso the day of the game, there's generally tickets: whether it's fans realizing johnny's swimming lessons are that day or it's a scumbag reseller who couldn't find someone to pay triple face, don't know. my guess is the latter. but you do see a scarcity of tickets, and then right beforehand, a glut. and oh, you will see some great tickets here sometimes. but the great seats are pricey, even at just face.
fyi, tickets at stubhub and ebay and what not are typically priced, as i mentioned above, about 2.5-3.0+ times face value. it's a real kick in the nuts when you're spending $65 for $19 bleacher seats.