Ulquiorra4sama said:
I'd recommend trying to find a surplus store in your town, most British towns have at least one, and I assume the same is true of America. Also, if you have an army base near you it may be possible to just go straight there and buy one. (Seriously, they basically hawk any surplus they have to make a little extra money.)
As for online stores, just type some variation of 'military/raf/army/polish/russian/british greatcoat' into google and you'll find one. (German and American greatcoats have a very different design to them) RAF are the blue that Jack wears, but sometimes you can get Naval greatcoats in that blue as well (the RAF alternate between blue and grey depending on officer/cadet/regular/non-com etc)
One of my uni friends got his grey RAF greatcoat by walking up to the base near his home one day and asking if they had any surplus jackets, he got it for £15. Lie I said mine was a military surplus store for £25. Online it'll cost you about £40+shipping. (Not sure about American online shopping, it's quite different.)