Barrak Obama posters?

So I would really like to buy a Barack Obama HOPE poster. But I don’t know a whole lot about posters. I want one from the campaign, not a cheap reprint. But, how can I tell the difference? how do i know if someone is telling the truth? (yes I know ebay but how do i know its not a knock off reprint) I search and i get overwhelmed with poster info, different sizes print this print that blah blah blah. Any insight would be nice thanks guys!