I too have a problem finding hats that fit - I've actually never found a woman's hat that fits - they are just all too small. But, I've also burnt my head and scalp on my first trip to Disney, so I started looking around for a hat that would fit.
The trick is to buy a hat that is sized - the generic baseball caps tend to be much too small.
Here are two websites that have GREAT hats - I have one from each of these, though my husband has stolen one of them from me - they are not cheap, but I've had one hat for 15 years, and another one for 10 years - so they really last a long time, and stay great looking (you can wash them, then block them to dry, and they look terrific):
Ultimate Hats
Tilley Hats
Both websites should have instructions on how to measure yourself to get the right sized hat - I would be very surprised if you couldn't find something that fit you.
One suggestion I'd make is if you're in Disney during the really hot months, to go with one of the tropical weight hats - the hat my husband uses is a standard weight, and I thought it was too hot to wear at Disney in August and September - but it suits him just fine! Everyone has their own tastes! I also like the ones that have a darker underbrim; helps deflect the glare from all the cement at Disney, but again, it's personal choice.
Good luck!!