For the person who mentioned jerseys101.com:
I had my own experience with them. Bought my wife a Roethlisberger authentic for her birthday (she's a Steeler fan)..
First and foremost, if you need this ASAP, don't order off jerseys101. It takes roughly about three weeks to get the jersey, partially because it's being shipped from Hong Kong.
I will tell you though, the price vs. the quality is well worth it and if you can wait, you're getting a hell of a deal.
Are these 100 percent authentic? Nope. However, unless you run over the jersey with a fine, fine tooth comb, you really won't know the difference one way or the next.
Some of these sites, you can get steals. However, just be wary and look at previous reputations, speak to people who have bought through them in the past and procede with caution. I've heard many stories about people who bought "authentics" that were horrendous knockoffs, with name mispellings, the wrong color (some one I know bought a Fitzgerald jersey, that had a Louisville Cardinal logo stitched on both arms and the color wasn't even close), or other horrible ripoffs.