Availability of the early varieties is probably somewhat dependent on where you live. But if you look hard enough, you should be able to find plenty of options.
I know that in Florida, Xie Shan budwood has only been available for a few years, so it is not likely to be widely available. Although some forum members have been able to order it from Harris Citrus in FL (and they are approved to ship out of FL now).
Stan McKenzie may have Armstrong Early, I purchased a tree from him ~3 years ago.
In Louisiana, LA Early and Early St. Ann should be relatively easy to find.
You can currently find St. Ann and Armstrong Early on Amazon via a middleman. I also believe there is another nursery in LA that might be willing to ship Early St. Ann, but the name escapes me at the moment.
Miho and Seto Satsumas may be available for shipping from RGV Nursery via the website citrustreesonline.com, but I do not have any experience w/ them.