I've never grown it, and don't know much about it, but Baker Creek does have seeds for it here
https://www.rareseeds.com/store/vegetables/garden-berries/strawberry-spinach. I was strongly considering buying some a while back, but then decided I have way more than enough seeds to plant, and I have to draw the line somewhere. I think I may have also gotten scared off because the listing said the seeds can be toxic in large quantities, so I would need to do more research into that. If my kids liked them, they'd be very prone to eating them in large quantities, and I just don't need the extra stress right now, haha. I find this plant really intriguing, though, because it is supposed to be heat-tolerant. I might try it next year.