If you can't wait, SmartSeeds has them available right now. I bought my C. procera seed from her last year, plus one or two others. 100% germination, she has some of the finest seed around.

My plants are returning from the brink of death (not too long a story...) now that the weather is warming up. If they set seed later in the year, you may have some.

In your zone, you will definitely need to overwinter them or else they'll freeze. From what I've read, they can freeze to the ground, but may sprout back up the next season.

A Milkweed Tree To Feed Them All