They don’t like hot weather, so depending on how “southern” you’re growing them, it might work better to grow them in the fall. They actually germinate better in cold soil, and in cooler climates the seeds overwinter easily, so sowing in the fall works for northern climates as well. Be aware that they reseed themselves very easily, so it would probably be best to have a dedicated spot just for poppies, instead of trying to rotate them like other crops.
As for where to get a starter kit, that depends on how specific you want to get. Varieties selected for opium production are illegal to grow in the US. But there are varieties selected for ornamental value, or for seed production, or both, that are readily available. You can find them just about anywhere that sells garden seed. Baker Creek has a nice range of varieties.
You can find more details, including some details that I refuse to type out, here: https://wimastergardener.org/article/breadseed-or-opium-poppy-papaver-somniferum/
Wow - lots f great info - thanks