This $60 RV water pump popped up on my Amazon recommendations:
It's a SHURFlo that looks exactly like the marine version
that West Marine sells for $150.
Obviously it doesn't have the corrosion-resistant coating and materials. But I'm only looking for a backup in case mine fails while I'm not somewhere I can get a new one quickly.
It lives in the engine room, where there isn't much problem with corrosion. And I'd be keeping it in its original box, inside a zip-lock bag, unless and until it's needed. I might not even need to replace my existing pump, just use the RV-grade spare for parts.