I maintain a gallery of split keyboards for people to see what's available: https://aposymbiont.github.io/split-keyboards/

Discussed here previously: https://news.ycombinator.com/item?id=26179311

Generally, hobbyist mechanical keyboards that you assemble by buying or fabricating a PCB, and doing a bit of soldering. Most designs have relatively easy soldering.

Some hobbyist shops may offer assembly services. If you don't want to solder, ZSA labs have a "moonlander" keyboard that looks quite nice, for a premium price.

If you're not put off by soldering, then some popular split keyboards would be the Corne (also known as "crkbd"), Kyria, lily58, Sofle.

There are also designs which are closer to a normal keyboard, like the ultimate hacking keyboard. (But, personally, I don't understand why you'd spend so much money on a keyboard and have it be asymmetric).

Surely the origins of split keyboards are ergo spaces not hobbyist?

Split keyboards have been around for quite a while. You can even see the Kinesis Advantage keyboard featured in films like Flubber and Men in Black and plenty of companies have made ergonomic keyboards.

I think there's a sort of hobbyist renaissance going on right now, though. Plenty of new keyboards are being designed and built because it's more accessible than ever to design your own pcb or if not that to get someone else's design printed, which along with the general mechanical keyboard enthusiast market, has given people a wealth of options to choose from.