Raw Threads has lots of cute themed shirts. Closer to the race, they will have lots of new options, many themed towards the specific races. For this past Princess weekend, you could even customize some of the shirts with the race distances. They are transitioning shirt lines on their website right now, so there's not much to choose from Princess wise, but there are some designs still available that will give you an idea of what they offer.
Their bamboo cotton is so soft and moisture wicking.