Where can I buy tickets for a football game?

So hello everyone. :) I am from Montenegro(http://en.wikipedia.org/wiki/Montenegro) and I'll be staying in London from 1st to 22nd August at the Queen Mary University of London for a summer language school. I am looking forward to see a match between Chelsea FC and Manchester Utd on 8th August. I hope that you guys can tell me which is the best (and cheapest) way to buy them. Thanks in advance :)

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