Ive found that the best places would be newegg and tigersdirect. You want to make sure that the place your buying from is legit of course. You could even check ebay.
in the uk its overclockers.co.uk. Otherwise try amazon.co.uk and ebay.co.uk. You will not find any really good ones though, as gaming cases that are any count start at about 60 USD and go up.