Bob in Orlando was looking for a replacement water pump for his
Westy...If you want a place that carries LOTS of pumps of ALL kinds, you
might try the W.W. Grainger Company.  They even have their catalog on
line, along with a search engine.  Find them at http://www.grainger.com  
I've seen 12v submersible pumps in that catalog.  Try looking for "RV 
pumps".

I've not tried ordering anything from them, yet, so can offer no comments 
about their service, etc.