I would advise going to a shop which has a wide range of different helmets, and then finding the one that fits the best. Different helmet manufacturers make different shapes and therefore some will fit better than others (same idea as shoes).
Look for something with Snell 95 certification if you can find it, as the test is tougher than the EN1078 that most helmets are submitted to, but fit is far more important than anything else. Also don't worry if the one you like isn't top of your price range, just go with what suits you best.