TicketMaster
TicketMaster is the best place to get tickets, if you can find what you're looking for. Their web site is hard to use because you can only search for one set of tickets at a time, and it takes forever. A lot of times they're sold out really fast.
www.ticketmaster.com
TicketLiquidator
TicketLiquidator is what they call a "ticket broker". Their prices range from 50% of the ticket value to 400% of the ticket value... depending on the game. It's kind of like an E-Bay for tickets, but it's a real business so you're not risking dealing with just some guy on the street who's going to rip you off.
www.ticketliquidator.com