Indeed, system76 seems to have ALL 15" models out of stock.
You could of course sign up and get notified when they get them again (as I'm sure they will evtl.).

Where do you live? entroware.com might be a viable alternative: https://www.entroware.com/store/laptops/zeus

There's one or two more such "alternative" laptop companies...
https://slimbook.es/
https://www.tuxedocomputers.com/
https://store.vikings.net/index.php?route=common/home

Have you also considered a Dell XPS laptop?

PS: one such company already went bankrupt because of Covid19, more or less... delays might have something to do with the pandemic. I recommend patience.