Eric, the Butterfly Retreat that Tyler mentioned is a good choice. Native American Seed also has some other good choices:
http://www.seedsource.com/catalog/detail.asp?product_id=4501
Host a party for butterflies at your place! Select a sunny area. These native annuals and perennials will do the rest. Next spring your Butterfly Retreat will invite your guests to feast on nectar and color throughout the season. Butterflies bring you dances of life that continue for years to come.
Contains a beautiful mix of species that will host butterflies in each phase of their life cycle.
These would be good choices also. Each link gives a list of the seeds in the mix:
Hummer & Singers Mix
http://www.seedsource.com/catalog/detail.asp?product_id=4503
Lady Bird's Legacy Wildflower Mix
http://www.seedsource.com/catalog/detail.asp?product_id=1814
Wildflower seeds are put out in October for a spring bloom. There are a few that can be planted in the spring. The ones I have done with success for a spring planting are:
Blue Sage, Indian Blanket aka Fire Wheels, and Purple Coneflower aka Echinacea purpurea.