1 single ticket may be hard to come by as they usually come in pairs when people try to sell them, but that is not to say that one's family member or friend may not be able to attend a game in the last minute, leaving a ticket available. Question is, will this person with the ticket available be willing to sit next to you during the game? 
j/k
Try looking on stubhub. I know stubhub is usually over priced, but sometimes when trying to find one single ticket, it may not be as expensive as you'd think. One single ticket is more difficult to sell than if it were 2,3, or 4 as fans are usually buying for their families or spotting for a group of thier friends attending the game. Because of that concept, the seller trying to pawn off that one ticket may have difficulty trying to get rid of it as it is not apart of a set and may sell it cheaper than normal. So check stubhub and see...it is worth a shot. Can't promise you'll find anything "cheap" as it is a redskins game at the final year at Texas Stadium, but it's certainly worth a look to see if you can find a single ticket that fits your price range.
And of course as mentioned, there may be someone on here willing to sell you their extra ticket if their friend of family member couldn't make it, but there's no guarantees. I think everyone wants to attend that game.