I've had the best luck in indie digital spaces, honestly. Sites like Smashwords and Draft2Digital host tons of self-published authors who don't have to follow mainstream imprint rules about what's 'too out there.' That's where you find the truly bizarre premises—like sentient furniture erotica or alien romance with truly non-humanoid anatomy. The search filters are terrible, though, so it's a slog.
You kind of have to embrace the chaos and scroll through a lot of dross. Sometimes the weirdest stuff has the most generic, bland covers and titles too, which is hilarious. I just finished one about a woman who falls for a ghost that haunts a specific subway car, and it was oddly poignant. Another was a merman romance where the central conflict was about interspecies marine biology conservation. No big publisher would touch those.
My strategy is to follow authors who write one weird thing, because they often have a whole back catalog of other weird things. Also, check the 'also bought' sections on those book pages; algorithm suggestions on those platforms can get delightfully unhinged.