You can still buy one from other retailers

But for how long?

Can't buy an unlocked phone from a carrier. Only other authorized resellers either don't sell phones (B&H, Adorama, Staples), have a limited selection which is in-store only (Target, Walmart), which leaves only Best Buy and right now I see everything on backorder. Now that Apple isn't taking new orders there's a good chance those will be cancelled.

And then there's the roulette wheel of buying from Amazon or Newegg where you might get a refurbished phone advertised as "new" if it isn't a literal brick in a box.

Still comes down to my fault for being slow to act. I said I wished for a warning but you know what they say about wishes.

(For context of how slow I am, I'd be upgrading from a SE (2016))

Any suggestions? I see Best Buy has some open box minis for $630 and there are plenty of 3rd party refurbs for around $500. Feels like I'd have been better off buying a brand new one for $600 yesterday.

I keep using Swappa, and it still works, even though no individual sellers are left.

Check the stores. I remember seeing old cycle products still for sale the week between announcement and the release date. Not necessarily true this time but you might get lucky.