Have you both checked Speeder Cycling? They have half a dozen options, I think the all road one could be what OG is looking for...

https://www.speedercycling.com/Carbon-Bike-Forks_c81

Good morning,
I tried a quick search but couldn't find anything on the topic.

I'm looking for a wide and deep (something similar to the Zipp XPLRs) set of 650b rims on a budget.
I've looked at the usual suspects (Farsport, Lightbicycle, Light Carbon, ... ) but I don't seem to find any.

I know people like to dislike this option, but I want to make a 1 bike - 2 wheelsets setup for my gravel bike and I'm missing a 650b wheelset for it.

It looks like 650b rims have fallen off favour in the last few years, but I do think that they make sense in certain situations, mine certainly, in order to maintain the same geometry when swapping from a 700c wheel with road tire to a 650b wheel set with a wide gravel tire + to maintain enough tire clearance with a 2x setup.

Any help is greatly appreciated.