The biggest fan I've ever seen is 140mm... I've never even heard of a 250mm fan meant for computer use... you'll probably have to search for hydroponic supplies to find something that big.
nevermind,now to this....
i am going to buy a 120-130mm fan, mount it on the frot of the case and put black plastic along the sides so it will fit in, and mount a black wire mesh or fan grill on it. how does it sound? here is my case. http://www.newegg.com/Product/Product.aspx?Item=N82E16811156135
You can get a 250mm case fan with an iCute case, not a well-known brand nor are they that popular but they have a 250mm fan as standard on the side, Maplins sell them.