Not sure if you're interested, but there's a cousin to Okinawa spinach, called Sissoo. Plants appear to be available here: https://sowexotic.com/products/sissoo-spinach-alternanthera-sissoo

Interesting. Thank you. Sissoo is a different genus but, like the others, does not produce seed. I must say that its rounded crunchy leaves sound more appetizing than the pointed leaves of gynura. Also, it seems to tolerate a lot of shade. In my fiercely hot summer, I defer to Malabar, which is nice, and thrives in the heat, but I'm looking to expand.

The problem with Sissoo and the gynura is that since you don't seed them, you can only preserve them by (1) protecting them from frost, which can be very inconvenient or (2) taking cuttings and growing them indoors until spring. Like the others, it's pretty pricey - $20 for ONE PLANT, including shipping, so preservation is advisable. Malabar is wonderful in that it self-seeds, and produces bazillions of seeds. Practically invasive.

Here's another source for sissoo but currently out of stock: https://www.herbcottage.com.au/brazilian-spinach.html

It's also on Amazon, common name: Brazilian Spinach.

And here: http://vanveenorganics.com/product/brazilian-spinach-alternanthera-sissoo/

Both of those latter two are in Australia. I am not. Shipping would be ridiculous. (Hard to tell from the Van Veen website, but you can look it up.) In fact, many of the purveyors of these plants are in the South Seas. I think Sowexotic is in Florida, however.

OK, I ordered and received two Sissoo cuttings from TheGreenhouseGuyShop, which is an Etsy marketer in Florida. Their price was much lower than others, so I was a little suspicious. But they arrived well packaged, and in beautiful shape. Each cutting was about five inches tall by five inches wide, with roots that strung down almost six inches. Probably rooted in water. I was impressed. I have potted them up and put them under lights for a few weeks to get ready to plant out. The leaves are dark green, rounded, and a little thick. Look beautiful for salads. Except they are kinda small. Only an inch or so across. But we'll see what happens as the plants mature.