Got it from an independent license vendor (https://www.license4you.de/windows-11-pro-neu?number=ID-1005..., but there's several).

I have no idea how they get the keys (probably OEM) and whether this is actually kosher (and I don't wanna know) but I did this with Win 10 some years ago as well and it has worked out thus far.

Just pay with e.g. PP and they'll send you the key via e-mail.

Grey market key resellers, apparently using volume licensing. It's a roll of the dice whether they'll stay valid, possibly because the same key is resold repeatedly, or Microsoft blacklists them.