As usual, Martha Stewart has an answer ;) Here is her guide to seasonal and regional flowers.
The Wedding Flowers Guide is a guide to seasonal, though not regional, flowers.
If you live in a region with a more limited growing season (ahem, the Rocky Mountains), try searching for wholesale flower retailers in your area, or call around to different florists. They will be able to help you decide what is in season and local.
Then you can arrange some of your flowers yourself, or all of them, or none! More on DIY flower arranging later. Or, consider alternatives to live flowers. Nettleton Hollow offers branches, dried flowers, fruits, and seed pods, and grasses. Bonus: keep them after the wedding to decorate your home, or send them home with guests.
Flowers by Saipua. Branches and dried flowers by Nettleton Hollow, via What Junebug Loves.