You can get tickets for face value at www.telecharge.com, but usually the best seats are sold out months in advance.
www.ticketliquidator.com is the biggest and (relatively) cheapest ticket broker on the internet. What that means is that they buy tickets the the show and then resell them for whatever they can get for them. While I don't like paying $250 for a $110 ticket more than the next guy, I have to say the convenience of having a huge selection and being able to choose front row seats on Friday for a Saturday night show is worth the extra cost. TicketLiquidator has the best customer service, too, which is nice because this is not really a customer-oriented industry.
Craigslist is sketchy and I don't recommend it. They do sell tickets for shows, though, so in the interest of creating a complete list of sources, I've added www.craigslist.org here. Caveat empor.
I hate being outbid, so I've kind of given up on Ebay. But they do have lots of tickets for sale and in theory you could get a great deal. When you're shopping for tickets, make sure you take a quick look at www.ebay.com.