Where can I find Washington Nationals tickets?

There are many tickets below face value at these sites:

http://craigslist.com (best for local, in-person purchases)
http://ticketstumbler.com (compares multiple ticket sites at once)
http://stubhub.com (largest ticket exchange)
http://ticketsnow.com (stubhub’s main competitor)
http://razorgator.com (high-end provider)
http://ebay.com (mostly auctions)