The Sox (maybe all of MLB, I don't know) use a lottery system. So you sign up for the lottery, and if you get chosen there is a specific time window that you have to opportunity to buy tickets in. I'm not sure when the sign-up for the lottery begins, but it can't be too long.
The cheapest way to get them is by winning the lottery, but it's unlikely that you'll win. StubHub will have tickets but they'll be very expensive.
Just looked at Springer's career line and it's very interesting. From ages 23 to 32 he was pretty bad (5.06 ERA, 1.49 WHIP, 1.97 K/BB ). Then I guess he had surgery in 2002, and from ages 34 to 40 posted a 3.51 ERA, 1.16 WHIP, 2.80 K/BB. Weird.