Daisy is the common name for many different plants. Shasta daisies are a type of Chrysanthemum and usually bloom in mid summer not late spring.

Bedding plants or perennials on sale now will be small and not likely to be thick and bushy by May. However, potted white mums and other white flowers that have been forced for early bloom are often sold for Easter and Mothers Day, they will have buds or be blooming.

You could buy those large, potted blooming plants and repot them into your boxes for the wedding.

Here is a link that might be useful: Shasta Daisy

Thanks so much! I'll keep an eye out when I'm in CA for blooming plants that I like :)