I've grown Asclepias tuberosa from seed and sort of by accident discovered that it did not require cold stratification. I received the seed from a trade and it was supposed to be A. curassavica, but the trader mixed up the seed. So I sowed it in April thinking it was tropical milkweed and that it didn't require cold stratification. I got lots of germination within a couple weeks, and after awhile figured out it was actually A. tuberosa!

I think you'd be okay starting it from seed now. Otherwise, you could try Prairie Moon nursery as they have bare root plants. Seed is much cheaper though.

I to never cold stratification mine and they alway come up too.
Angie

Rose Franklin perennials has them pretty cheap.I bought agastache and red hot poker from here and she gives healthy large plants for a very cheap price.