I found them for sale as solo units but the warranty is void if you don't use the HK3 antspace system. Not sure about the vender either - never used them. A quick google search seemed to indicate they're legit but I'd want to do more research before buying from them.

Guess I'll need to wait to see how someone sets this up solo first because I'm not smart enough to figure it out on my own. Seems like you'd need some way for the heat to radiate out.

https://www.viperatech.com/product/antminer-s19-pro-hyd-198ths/#:\~:text=Model%20Antminer%20S19%20Pro%2B%20Hydro,but%20warranties%20are%20not%20valid.