http://www.milkweedfarm.com/
Milkweed farm has all the info you need, plus seeds, too.
If you can find common milkweed in the wild in a location that you are not trespassing (no taking plants from parks), you can dig up the root and transfer it to your garden. This plant runs underground and pops up wherever it wants. You can keep it down by mowing, but if you are into Monarchs, the more milkweed the better. Starting common milkweed by rhyzome is the easier than by seed.
I have loads of common milkweed seeds, I can share if you want some.