Canadian looking to buy tickets to an EPL soccer game

Folks, we will visit London in early December and we would like to see the West Ham United vs. Manchester United game on December 5th

I checked out both West Ham United and Manchester United's official websites and the tickets for said game are not available for sale yet...or maybe I am not looking at the right places?

I googled and there is a ton of results but I don't know if those sites are legit.

Can someone let me know how to go about getting tickets for this game please? Should I have the tickets delivered to my residence here in Canada or should I pay now and pick them up at the ticket office at Upton Park?

I know the game is still 4+ months away but I just want to make sure I can grab some seats without paying an arm and a leg or worse, getting ripped off.

Thanks a lot.