Here's a blog post where someone attempts to process and spin milkweed stalks. Keep in mind, milkweed stalks produce a bast fiber whereas milkweed fluff can also be spun but makes a brittle yarn by itself and is often mixed with wool or cotton, though I would assume you could process the stalks and then spin the floss into the fiber from the stalks. Spin Off talks about it here. Milkweed also sustains monarch butterflies, which have been having a rough time. You might have some skin reactions when handling milkweed though, so keep an eye.
Fibershed did a spotlight on dogbane, nettle, and milkweed in comparison to hemp and flax. They talk specifically about Native American techniques and current cultivation of all three, particularly dogbane, and ways to spin yarn and make rope. They aren't super in-depth about processing the fibers, but it seems like it's because they have more tutorials/guides on other pages. Bast fibers all have a similar retting->drying->scutching->hackling->spinning (you also have to ripple flax at the beginning) process, generally speaking.
Of course, flax is a really accessible plant fiber to spin yourself! You can plant it yourself, buy unprocessed flax (locally or online), or buy processed but unspun flax. If you're interested in the whole process, this book is the flax bible: Linen: From Flax Seed to Woven Cloth by Linda Heinrich. There are a lot of youtubers who go through it all, too.
If you're interested in kudzu, then may I suggest Oigawa Kudzu-fu Studio! They're located in Japan, but they have helpfully created an English website, too, where they go over making kudzu-fu (Japanese name for the cloth from kudzu). And if you ever find yourself going to Japan, you can take a class from them.
On Oigawa Studio's info page, they also mention that they work with ramie (plant in the nettle family), linden tree (basswood), and wisteria, so do with that what you will. I assume a lot of those resources are in Japanese.
Finally, hemp. Laws around if and how you can plant hemp, even for personal non-THC uses, vary wildly from state to state and country to country, so do your research. I'm not super familiar with it, and in my state I literally can't grow any of it at all without applying for a permit, so it's not really worth it for me.
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