Be careful using boxed mixtures-- you'll get better results from a native mixture that has plants local to your area. If you call your local Native Plant Society or Wild Ones chapter, they should know where to buy seed-- they might even have some extra to share.

Here is a link that might be useful: Wild Ones chapters

We have had such great results from wild flower box mixes entire quarter acres of our land are bursting to the brim with the most gorgeous pot marigolds, lavateras, wild snaps ans cosmos. Don't sow to deep though and overwatering after established can make them rangy. Good Luck!