Are you in the US? The somewhat well known mail order nursery Mendocino Maples, in northern California as the name would imply (!), appears to offer it for sale, but their website is being revamped at the moment. I've never ordered from them but they appear to have + reviews.

Never heard of it, but, wow, no less an authority than Roy Lancaster calls it his "favourite maple ever!"

https://www.treesandshrubsonline.org/articles/acer/acer-micranthum/

Thank you, davidrt28!

I've been trying the nurseries and I haven't had any luck yet with any in stock. I'll keep searching...