The problem is that polls at this point are pointless - they have zero outlook on the election. Most voters don't tune in until much closer to the election, if ever.
Given that, TradeSports has a spin off that is a futures market for a variety of things, including the US presidential election. But it seems hard to believe that Obama is twice as likely to win as McCain. OTOH, they picked McCain to win the GOP nomination way back when, and that also seemed really unlikely. Unlikely to me, anyway. But here we are, they called it.