Before engaging in this hobby (monarch rearing) my favorite hobby was native landscaping, and I've learned a couple of lessons about milkweed from the one that apply to the other.
One, it's usually worth the wait to start from seed with most varieties, rather than transplanting. Many milkweeds don't transplant well, and some not at all. The few times I've had any success transplanting common milkweed, the plants survived a couple of seasons but never thrived.
The exceptions seem to be the ones most people say they do find in garden centers, swamp milkweed (often sold as "red milkweed") and butterflyweed. These transplant well and are super-showy, and that's why nurseries raise them.
Common milkweed is still considered, um, a weed in the nursery trade as well as by far too many agricultural and extension agents.
I'd like to make a plea for some other native milkweeds that are exceptionally attractive to the eye and incredibly attractive to monarchs, but pretty much have to be grown from seed.
In the upper Midwest (I'm writing from Wisconsin), two fabulous milkweeds, purple milkweed (Asclepias purpurescens) and Sullivant's milkweed (A. sullivantii) are both endangered and beautiful. Plus, they don't spread nearly as far and rapidly and unpredictably as common milkweed.
Planting these milkweeds not only helps monarchs, it helps preserve and restore important species of the milkweed genus that are disappearing from the landscape.
My favorite seed source is Prairie Moon Nursery near Winona, Minn. They have seed of both these and a half dozen other Asclepias species.
So, wherever you live, don't just rear monarchs, rear milkweed. Find out what your region's native species are and grow them from seed from the nearest source. You'll have better-adapted plants and a great sense of accomplishment, even if it takes a season longer.