You might have a rough time finding a legit authentic at this point. Since Reebok is out at the end of the season, they're just just selling what they have left and not producing to meet demand.
As far as finding a legit one goes, just use common sense. The real ones are close to $200 so anything significantly cheaper than that is probably a fake. The old adage "if it seems too good to be true, it probably is" applies here. The vast majority of jerseys on ebay and sites that only sell jerseys are fakes. Start with the bigger time retailers and work your way down.