Hi Christine:
Great idea. Who doesn't love Mickey Ear Hats? You have a many options where you can pick up Mickey Ear Hats.
If you want to arrange the ears prior to your trip, you can check out the ear hats for sale on the Disney Store
online, many of which can be personalized. As of now, there do not seem to be Cars-themed Mickey ear hats for sale on the Disney Store online.
You will also find Ear Hats for sale at just about every store throughout the parks and resorts. I've seen Lightning McQueen Ear Hats for sale in the parks in the past, but remember, selections are always changing. Your best bet would be to visit the Create Your Own Ear Hat Station in Downtown Disney, located next to the Art of Disney and Design-a-Tee stores. Other stores with large selections of Mickey Ear hats include the Chapeau Hat Shoppe on Main Street in Magic Kingdom, Sir Mickey's in Magic Kingdom, Mouse Gear in Epcot, and the World of Disney Store in Downtown Disney.
Enjoy your vacation.