I always look for one that matches my enormous bounce and is head-shaped:
then I call Luke at Playskool with my dimensions, and he sends out a fantastic helmet that complies with MSA standards for a very fair price.
Call Luke and talk about your requirements; he'll make sure you receive what you need.
I have to wear a helmet to work, and I'd advise spending more money to acquire one that fits and is comfortable rather than one that just looks beautiful.
In the last two years, my helmets have saved my head from serious harm on two separate instances.