Hi Adam!
I'm just up the road in Tampa, and while I don't have Black STs yet this year, I do get them every year. I grow Rue for them, although I've volunteered at MOSI where we grow parsley, dill, and rue. Fennel works, too. I was at Selby Gardens a couple of weeks ago for the Chagall exhibit and they had Black ST cats on their parsley, so they should be in your general area.
You don't have to buy organic seed. It's really more important that you don't spray the plants with anything. As an aside, the most commonly used pesticide to kill caterpillars is BT spray which is considered organic because it's a naturally occurring bacteria. They've just concentrated it into a spray.
Most of our conversation about butterflies takes place in our monthly threads. We post our pictures, but we also discuss attracting and raising butterflies, among other things. Feel free to pop in and ask questions. We've got a lot of members with lots of opinions!
The thread "April 2017 Butterflies, Moths & Larva" in
Gardening for Butterflies, Birds and Bees forum