I've had luck with baseball and football jerseys from Modells and Kohls. Kids sizes. I've paid $20 for a Polamalu and Hines Ward jerseys for nephews and my son for Christmas from Kohl's. (They offered free shipping on orders over $50 at the time, so I threw something in to make it $50, otherwise it would have been over $50 with the cost of shipping.)
I think we bought my son's Grady Sizemore and his Roethlisberger from Modell's. They've always had great prices. (I think the Grady was $35 and the other closer to $50--after they won the superbowl, nothing was cheap with Steelers on it.)
I think all the Steelers stuff was Reebok. I'm trying to remember what the Sizemore was. I know there was no way I was going to pay $$$ for game quality when they outgrow it so fast.