My DD3 LOVES cars and I have found very little for girls. When the movie first came out I'd find thigns with Sally on them from time to time, now I find very little.
So, I let her buy boys T-shirts and grandma has made her a shirt from Cars fabric (found an very easy pattern for summery, girly shirts) as well as a quilt (very cool-she found checkered flag material).
I have made my own iron ons from clip art and that special iron on transfer paper for ink jets and I have also cut out individual cars from fabric and used wonder under to make iron ons for some plain T-shirts (okay, I think they would be considered appliques)...then I sewed around the edges to make sure they stayed in place.
I am continually disappointed in not being able to find girl cars shirts (Disney are you listening???) My little girl always seems to like "boy" things (Cars, Transformers, Diego). SO she has become a T-shirt & jeans sort of gal already.