You are generally fine on ebay with things like caps, these aren't faked in great numbers. It sounds like we bash ebay alot here and looking at it's poor record in stopping counterfeiters it deserves it, but i really love ebay. With a little bit of research and some patience you can find the real deal on jerseys. Use the legit and questionable sellers lists here to check if they sell fakes or not.
The 'Thumbs Up' scheme we run has a few really good UK retailers, which I always recommend as we have checked that they only sell genuine goods.
ta-sports.net have a good range also.