don’t you think he should have spares lying around?
Not really, at least not identical ones. We’ve seen that there doesn’t seem to be a standard issue Knight of Ren helmet, everything looks to be either scavenged or custom built. If Kylo found his helmet somewhere, then I doubt he’d have another one just like it, and if he built it, it would be a bad narrative choice to have him craft a new one that’s exactly the same as the old one after he’s lost it in this life changing experience. I think it’s possible he’ll end up with something very similar, especially if it was an existing piece that he repurposed rather than something he forged himself, but I think it will be tweaked to some extent. Think of all the variations of Mandolorian armor we’ve seen. He could come back with something that’s clearly from the same culture or order or whatever, but pretty obviously not the exact same object.