You could try the ticket exchange scheme that Arsenal operate, although I'm not sure if you have to be a member at some level to take advantage of it.

If you will be in London on 30th/31st July, they host The Emirates Cup, 2 days, 2 games per day with 4 teams competing for the trophy. Tickets go on general sale around the 19th May so you could get lucky. Tickets are up to £29 maximun and you get to see 2 games for that, the 2nd one being Arsenal vs NY Red Bulls on the Sunday, with a certain Mr. Henry playing, who will, I think, play a half for each team

I know all this because we just bought 2 tickets for the Sunday games for my daughters, & I'm gutted as I'll be on a beach in Mexico at the same time thank goodness for Sky+

Lady1964