There's also a fairly new subreddit, /r/ErgoMechKeyboards/, which only has ergonomic keyboards (simply split ones, or more usually with staggered columns rather than rows, and often a 3D shape).
Fewer fancy, expensive keycap sets, and more works-in-progress with wires sticking out.
There are small companies selling prebuilt keyboards and kits elsewhere, but I don't know them offhand, except for https://www.aliexpress.com/item/33006060286.html from China (as I have one). There's also https://www.reddit.com/r/mechmarket where there are people that will build the keyboard for a fee, or sell keyboards second hand.
I use a kinesis advantage2 with brown cherry keyswitches. (I think it's available with others) - my only complaint is that it's kinda giant.
Overall, it's pretty great; if you don't want to build your own, and if you don't want to change your keyboard, it's the best choice because it has been in production since the early '90s with few changes to layout; for me? this is big- changing layout is a big deal. The kinesis has been in production long enough that I am pretty confident I won't have to learn a new layout until keyboards go by the wayside
It's probably the least portable keyboard I've owned, though.
I use the Advantage2 as well, and now have two so I don't have to lug one around to my second location. But when I got my first one, it sat for almost a year because I couldn't be bothered to learn how to type on it. I'm glad I finally took the plunge though because it has relieved my hand/wrist problems and I love typing on it.
Most recent models can only be found in kits or even parts you need to source. Some people sell assembled keyboards so you can limit yourself to those, but at this point your best bet is to buy a kit or the parts and find someone to assemble it. It's really not difficult to do if you can solder and have a bit of patience, if that's a design with a PCB. Handwiring is more challenging.
One model that's easy to find assembled is the Ergodox. But it's been improved by new models like the Iris, Corne, Gergo, Kyria, Lily58…
I've been using a Kinesis Freestyle Pro for over a year. The wristpad they sell is pretty bad, so I attached the wristpad from my Matias Ergo Pro (the keyboard kept breaking).
Fewer fancy, expensive keycap sets, and more works-in-progress with wires sticking out.
https://www.reddit.com/r/ErgoMechKeyboards/