Odds are, you didn't get tickets to Book of Mormon—opera donors swooped up a ton of tickets in presale, and as soon as tickets went on sale to the general public they sold out more or less instantly.
You'll be able to pay too much for tickets on Craigslist, of course, but there's another option: For every show, Broadway Across America releases a handful of $25 tickets, distributed by lottery. Here's the details:
Entries will be accepted at Keller Auditorium (SW 3rd & Clay) box office beginning two and a half hours prior to each performance; each person will print their name and the number of tickets (1 or 2) they wish to purchase on a card that is provided. Two hours before curtain, names will be drawn at random for a limited number of tickets priced at $25 each. Only one entry is allowed per person. Cards are checked for duplication prior to drawing. Winners must be present at the time of the drawing and show valid ID to purchase tickets. Limit one entry per person and two tickets per winner. Tickets are subject to availability.
That sounds much more civilized than when my best friend and I camped out at the "Civic" for Rent tickets in high school... twice :/
Book of Mormon is playing at the Keller Auditorium, 222 SW Clay, Tues Jan 1-Fri Jan 4, 7:30 pm, Sat Jan 5, 2 & 7 pm, Sun Jan 6, 1 & 6:30 pm, $25, cash only