Where can I buy Washington Nationals tickets?
Here's the July schedule:
Any games in red on the official page are home games. By clicking the little "T" on a specific date, you can go through the process to buy tickets. The price is the same as at the park, except for the annoying $3.50 ticketmaster charge per ticket.
Depending on the game, this site has some coupon codes that may work. You put in the code in ALL CAPS on the official ticket purchase screen at the top (after you get there by clicking "T" on the calendar). I have no association with the website but got discounted tickets for this Wednesday's Strasburg game by typing EARTH in the promotion box. Unfortunately for you, this promotion appears to end this week.
Of course, StubHub has tickets too.
As long as you are going to a non-Strasburg game, plenty of seats should be available the day of the game, though you won't get a discount (except for not paying the "convenience" fee from Ticketmaster). Good luck.