Regional selections of native wildflowers should suit you fine.
Most distributors have half a dozen or so regional mixtures.
These are usually both annuals and perennials. (The annuals should reseed themselves.)
Most of these mixtures have early, mid, and late blooming species.
A huge advantage to these mixes is that they provide food and habitat for the native pollinators.
An example of one of the many distributors:
http://www.edenbrothers.com/store/wildflower_seed_mixes.html