Welcome to the never-ending frustration of finding native plants. Unless you can find a specialty nursery nearby, I'm afraid that you must resign yourself to mail-order nurseries.
I did a web search for Phlox buckleyi, and found one;I'm sure that there are others, and maybe one will turn out to be close enough to get to.
Good luck!

ps-I know nothing about this nursery, so you should check it out on Garden Watchdog

Here is a link that might be useful: nursery with Phlox buckleyi